The expression of Ia-like antigens on human colony forming units-granulocyte macrophage (CFU-GM) is related to S-phase of the cell cycle, and associated with the control of normal granulocyte and macrophage production by prostaglandin E and acidic isoferritins in vitro. Ia-antigen expression by CFU-GM is lost within 3-6 h of culture at 37 degrees C and occurs simultaneously with loss of responsiveness to inhibition by these factors. Culture of bone marrow CFU-GM in a limited exposure suspension culture with 1 microM-1pM prostaglandin E (PGE1 or PGE2), but not prostaglandin F2 alpha or dibutyryl-cyclic-3'-5'-AMP results in the detection of CFU-GM Ia-antigen after 24 h. Antigen expression is associated with an absolute increase in total and S-phase CFU-GM, and restoration of responsiveness to inhibition by prostaglandin E and acidic isoferritins. The detection of Ia-antigen on CFU-GM after suspension culture with prostaglandin E results both from Ia-antigen reexpression as well as stimulation of noncycling cells to enter S-phase, express Ia-antigen and give rise to CFU-GM sensitive to inhibition by prostaglandin E and acidic isoferritins. The sensitivity of CFU-GM to inhibition by these factors after suspension culture with prostaglandin E is identical to that of the same cells tested prior to the suspension culture. These studies provide evidence for a direct regulatory association between Ia-antigen expression and control of myeloid progenitor cell differentiation, and suggest a role for prostaglandin E in the control of CFU-GM cell cycle, Ia-antigen expression, and growth regulation.