The influence of prostaglandin E1 (PGE1) on colonies produced by different subtypes of committed granulomacrophagic precursors (CFU-GM) was studied. Even though they represent a more immature class of precursors than day 7 CFU-GM, day 14 CFU-GM have the same sensitivity to PGE1: their growth is significantly inhibited by low PGE1 concentrations (10(-10) M or higher). Macrophagic colony formation is most sensitive to PGE1. Pure granulocytic colonies are relatively resistant and significantly inhibited by high concentrations only. Mixed granulomacrophagic colonies display an intermediate degree of sensitivity. Both day 7 and day 14 CFU-GM from chronic myeloid leukemia and idiopathic myelofibrosis patients are significantly less sensitive than normal CFU-GM. This insensitivity is not dependent on a lack of production of macrophagic colonies, since these may even be present in a higher than normal frequency.