Preincubation of C57BL adult marrow cells or CBA fetal liver cells with a 250‐fold excess concentration of purified GM‐CSF failed to reduce the frequency of cells forming eosinophil, megakaryocyte or erythroid colonies in subsequent agar cultures. When excess concentrations of purified GM‐CSF were added to agar cultures stimulated by pokeweed mitogen‐stimulated spleen conditioned medium (SCM), no reduction was observed in the frequency of eosinophil, megakaryocyte or erythroid colonies. Addition of 4 units of purified erythropoietin (EPO) to cultures of fetal liver or adult marrow cells stimulated by SCM increased the number of erythroid colonies but did not reduce the number of non‐erythroid colonies or the non‐erythroid content of mixed erythroid colonies. Although neither GM‐CSF nor EPO alone was able to stimulate erythroid colony formation in agar cultures of fetal liver cells, small numbers of large erythroid colonies were stimulated to develop in cultures containing both purified regulators. Purified GM‐CSF was also able to support the survival in vitro of a small proportion of erythroid colony‐forming cells in fetal liver populations cultured initially in the absence of SCM and the survival of some eosinophil and megakaryocyte colony‐forming cells in similar cultures of adult marrow cells. The results do not support the hypothesis that GM‐CSF and EPO compete for a common pool of uncommitted progenitor cells. On the contrary, the data indicate that GM‐CSF and EPO are able to collaborate in stimulating the proliferation of some erythropoietic cells. Furthermore, purified GM‐CSF appears to be able to support temporarily the survival and/or initial proliferation of at least some cells forming erythroid, eosinophil and megakaryocyte colonies, even though GM‐CSF is unable to stimulate the formation of colonies of these types. Copyright © 1979 Wiley‐Liss, Inc.
