A modest Midwesterner who became one of the most influential geneticists of the first half of the 20th century, William E. Castle spent his career at Harvard and the University of California working on patterns of inheritance in mice, horses, and a variety of other mammalian taxa. An early proponent of Mendelian theory, Castle was director of the Bussey Institution at Harvard for almost thirty years, helping to train a number of important geneticists. The Castle Papers contain one linear foot of correspondence dating primarily from the period after Castle's "retirement" to Berkeley in 1936 until his death in 1962, dealing almost exclusively with his research on horse breeding and the inheritance of coat coloration in horses. Castle's correspondence with his former student L. C. Dunn is an exception, focusing on mouse genetics and ranging to a variety of topics from the conduct of scientific research during the Second World War to Castle's interests in the early history of genetics.

The geneticist Milislav Demerec emigrated from Yugloslavia in 1919, spending nearly his entire career in the Department of Genetics at Cold Spring Harbor Laboratory. Working initially on the genetics of maize and later on Drosophila virilis, his research interests included radiation and chemical mutagenesis, and during the Second World War, penicillin and the genetics of antibiotic resistance. Demerec served as head of the laboratory at Cold Spring Harbor from 1941 until 1960. The Demerec collection contains the extant professional papers of Milislav Demerec, dating primarily from the time of his arrival at Cold Spring Harbor until his retirement. In addition to his correspondence with colleagues, the collection includes interesting material on the administration of Cold Spring Harbor laboratory during the 1940s and 1950s, data and research notes, material relating to professional organizations (e.g. Genetics Society of America, American Society of Naturalists), a lengthy series of lectures given by Demerec, and a large number of photographs pertaining to Demerec's research, but also to his colleagues and Cold Spring Harbor itself. There is also a Register of visitors to the laboratory (1927-1945) and a Milislav Demerec reprint list.