Mostly brief correspondence with individuals but includes some larger runs of correspondence.

There is a series of correspondence with and material relating to J.B.S. Haldane (MS. Darlington c. 108/J.73-89) which comprises correspondence and writings by Haldane (MS. Darlington c. 108/J.73-78), and drafts, articles, press cuttings and correspondence by Darlington and others, mainly after Haldane's death in 1964 (MS. Darlington c. 108/J.79-89). Haldane was at John Innes from 1927 to 1936, as 'Officer in charge of Genetical Investigations', a post he combined with a Readership at Cambridge (to 1933) and Chairs at London. Darlington was at this time 'cytologist' at the John Innes. He and Haldane became personal friends, as can be seen from diaries and autobiographical writings, and they published a collaborative paper (with P.C. Koller) in 1934. Haldane had expected, and been expected, to succeed A.D. Hall as director, but Hall's resignation was delayed. Haldane resigned in 1936 and Darlington became director in 1939. See Memoir, pp.118-121, and the Memoir of Haldane by N.W. Pirie, Biographical Memoirs of Fellows of the Royal Society, 12, 1966, and passim in the material below.

There is extensive correspondence with P.C. Koller a life-long friend and colleague of Hungarian origin (MS. Darlington c. 110/J.122-139). He worked at the Institute of Animal Genetics, University of Edinburgh in the 1930s with several visits or longer periods of work at the John Innes. In 1944 he moved to London as Research Cytologist at the Royal Cancer Hospital (1944-1946) and the Chester Beatty Research Institute (1946-1969), becoming Professor of Cytogenetics, University of London. He died in 1979. Correspondence concerns research, publications, visits and similar aspects of professional life. There is, however, a 'personal' element deriving from Koller's volatility of temperament and from the many problems he initially found in adapting to life in Scotland. The letters are usually dated; some undated material has been tentatively assigned on grounds of content. In August 1939 the Seventh International Congress of Genetics was held in Edinburgh. Darlington was the Recorder and Koller the Secretary of the Cytology Section. The Congress looms large in the correspondence for 1938-1939 since the possibility of war added to the already considerable difficulties attendant on the participation of Russian and German scientists. In the event, the Russians withdrew, and many of the Germans left early. Koller continued to use up stocks of the 1939 Congress writing-paper well into 1941 and this adds to the difficulties of dating. The folders include letters from other colleagues working or in touch with Koller at a particular time. There are few letters from Darlington, but frequent manuscript comments by him on the incoming correspondence. Koller usually signs himself, and is addressed as, 'Pio', but sometimes the form 'Peo' is found.

There is a short series of correspondence with, and relating to, W.C.F. Newton (MS. Darlington c.112/J.167-170), 1925-1929. Newton was one of the earliest scientific influences on Darlington. He had been appointed by Bateson as cytologist at the John Innes in 1922, only a few months before Darlington, who worked under his guidance until his illness in 1926 and death in 1927. They published several collaborative papers, some posthumous. In 1925 Newton married Lily Batten, herself a distinguished botanist. There is correspondence with Newton and his wife, and from Darlington's parents and members of the John Innes staff who kept him posted about Newton's declining health during his visits abroad.

There is a small amount of material (MS. Darlington C.114/J.223-226) relating to Nikolai Vavilov, a great Russian geneticist, who had worked at the John Innes with Bateson in 1913 and 1914. After his appointment by Lenin in 1921 to the joint post of President of the Academy of Agricultural Sciences and director of the Institute of Applied Botany, he was an influential figure in Russia and also in international scientific circles. In 1939 he was to be President of the Seventh International Congress of Genetics (held in Edinburgh) but the Russian delegation did not attend. Vavilov fell victim to the rise of Lysenko and the acceptance by Stalin of the latter's theories; he was arrested and sent to Siberia, where he died in 1943. An English version of his last work was published in 1940, largely through Darlington's good offices. Vavilov was elected a Foreign Member of the Royal Society in 1942.