The in vitro expression of MHC class II Ag by human bone marrow colony-forming unit-granulocyte and macrophage (CFU-GM) and their proliferative response to negative growth regulators in agar culture are transient and can be modulated in vitro by 24-h suspension culture in the presence of PGE. Analysis of the participation of accessory cells in this phenomenon indicates that the ability of PGE to modulate CFU-GM MHC class II expression, the proportion of CFU-GM in S-phase of the cell cycle and the responsiveness of CFU-GM to inhibition in vitro by two negative growth regulators, acidic isoferritin inhibitory activity and PGE itself, requires the participation of CD8+ T lymphocytes. This effect is mediated by a lymphokine of m.w. 27,000 that we have purified to apparent homogeneity. This lymphokine possesses neither colony stimulatory nor inhibitory activity, is produced by both peripheral blood and bone marrow CD8+ T lymphocytes, as well as the CEM T-ALL cell line, and requires the obligatory presence of PGE for activity.