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.
The Lesser mss., 1933-1971, consist chiefly of the correspondence between Sol Lesser, 1890-1980, motion picture executive, and Upton Beall Sinclair, 1878-1968, novelist, and others concerned with their interests.
The Lush mss., 1956-1957, consists of two letters from Upton Beall Sinclair, 1878-1968, author, and a letter from Mrs. Mary Craig (Kimbrough) Sinclair, (Mrs. Upton Beall Sinclair), 1883-1961, author, to Samuel Beryl Lush, 1893?-?, cotton merchant.
The Saturday Review of Literature mss., 1946-1956, consists of letters from author Upton Sinclair, 1878-1968, to Norman Cousins, 1915-1990, and Harrison Smith, 1888-1971, editors of the Saturday Review of Literature.
The Stokes mss., 1905-1906, consist primarily of letters from Upton Beall Sinclair, 1878-1968, novelist, to James Graham Phelps Stokes, 1872-1960, regarding the activities of the Intercollegiate Socialist Society.
The Viereck mss., 1943-1953, consists of typescripts of letters from novelist Upton Beall Sinclair, 1878-1968, to Peter Robert Edwin Viereck, 1916-2006, author and professor of history at Mount Holyoke College.