This series contains correspondence (most incoming, arranged alphabetically) with like-minded communists, ex-communists, other U.S. leftists, historians and other researchers, and is primarily concerned with communist history and with current events, radical thought and activity. There is also some family correspondence. Additionally, several letters relate to Green's support of organized labor and independence movements in Puerto Rico in the late 1970s and early 1980s. Prominent and/or principal correspondents include: Harry Bridges, Lloyd Brown, David Engelstein, Sender Garlin, Dorothy Healey, Kate Hyndman, Lolita Lebron and other Puerto Rican radicals, Sidney Lens, Helen North (his second wife), Paul Robeson Jr., Bill Schneiderman, Pat Toohey, Saul Wellman and Leon Wofsy. Folders labeled "Addendum" were moved into this series in 2016, following the consolidation of materials into Series I and Series II.

This series consists of copies of (mostly outgoing) letters between Gil Green and members of his family during his time in prison. The majority of the letters are addressed to Green's first wife, Lillian Green, and their children. There are also several letters between Gil Green and his brother Ben. These letters predominantly relate to personal issues, but also touch briefly on political topics in relation to Green's trial and sentence.

This series contains unpublished typescripts by Green and others, notably James Allen's memoir Marxist Publishing (n.d., 44 pp.), A Labor Revolutionary in Detroit (re Nat Ganley), and Green's 1968 notes on his remarks on Czechoslovakia at the CP's Central Committee meeting thereon; editorial files pertaining to the writing, publication and reception of several of Green's later books, notably his autobiographical Cold War Fugitive (1984); third party correspondence of several of Green's political friends and associates, notably Lloyd Brown; and a file of Smith Act-related material obtained through Freedom of Information Act requests.

This series makes up the bulk of the collection and contains all of the files sent to Gil Green from federal agencies between 1978 and 1991 in response to his numerous Freedom of Information Act requests. They consist of FBI reports, correspondence between bureau offices, transcripts of trial proceedings and testimony, surveillance logs, and copies of documents produced by individuals and organizations under FBI surveillance. A large portion of these files deal with the FBI's hunt for Green between 1951 and 1956. During these years, Green was a political fugitive seeking to escape federal charges for violating the Smith Act. The series also contains a number of FBI files obtained through the efforts of Gil Green's attorney, Edward Greer, and contain FBI files from various regional offices and files on individuals affiliated with Green or the CPUSA between the 1930s and 1970s. FBI files contained in Subseries IVB were grouped by Green in preparation for his 1984 memoir, Cold War Fugitive: A Personal Story of the McCarthy Years. These files are heavily annotated and reflect the various stages of the FBI's campaign against Green. They do not, however, correspond with chapters in his book.

Along with FBI files, Green received several hundred pages of documents from the Bureau of Prisons and Parole Board. These files contain copies of Green's personal notes and correspondence during his time in prison, as well as conduct reports, telephone logs, and parole board paperwork.

Also included among these materials is a small amount of correspondence between Green, his attorney Ed Greer, and various federal agencies regarding Green's FOIA requests between 1975 and 1981, held in Subseries IVA.