The Colorado-Sinclair mss. consists of letters exchanged between author, Upton Beall Sinclair, 1878-1968, and University of Colorado Director of Libraries Eugene Holt Wilson, 1909-?, Boulder, Colorado, August 25-October 31, 1950.
The DeWitt mss., 1941-1955, consists of letters from Upton Sinclair, 1878-1968, author, to Samuel Aaron DeWitt, 1891-1963, poet and businessman, and correspondence between Mary Craig (Kimbrough) Sinclair, (Mrs. Upton Sinclair), 1883-1961, and DeWitt.
The Eeden mss. consists of two reels, 1908-1913 and 1914-1918, of negative microfilm of letters and postcards of Upton Beall Sinclair, 1878-1968, novelist, to Frederik Willem van Eeden, 1860-1932, poet and dramatist, from the originals in the Frederik van Eeden-Genootschap at the University of Amsterdam in Amsterdam, Holland.
The Haldeman mss. III, 1915-1918, consists primarily of correspondence of Upton Beall Sinclair, 1878-1968, novelist, and Appeal to Reason staff members Louis Kopelin, 1887-1949, and Walter H. Wayland, 1884-1980.
The Howatt mss., 1902-1959, consist in the main of letters to David Howatt, 1882-1964 , farmer and secretary of Upton Beall Sinclair, from writers Harry Kemp, Upton Beall Sinclair, and William James Durant.