“…Cell Separation.--Discontinuous bovine serum albumin (BSA) density gradients (34) were used to separate thymocytes into four bands, designated A, B, C, and D (from least to most dense) plus a pellet, P. As in earlier work (35,36), larger cells were found almost exclusively in A, B, and C, and macrophages in A and B. For removal of adherent cells (2, 6), 1-ml all quots of cell suspension, containing l0 T cells in MEM with 10% normal human serum or fetal calf serum (Grand Island Biological Co., Grand Island, N.Y.), were incubated in Petri dishes (No.…”