Multipotential FDCP-Mix A4 (A4) cells can be induced either to self-renew or to differentiate and develop into mature neutrophils in liquid culture, depending on the haemopoietic growth factors with which they are cultured. When cultured in low concentrations of interleukin 3 (IL-3, 1 unit/ml)) plus Granulocyte Macrophage Colony Stimulating Factor (GM-CSF) and Granulocyte-CSF (G-CSF), A4 cells proliferate with accompanying development to form cells which resemble mature, postmitotic neutrophils. The presence of high concentrations of IL-3 (100 units/ml) blocks the development of A4 cells even in the presence of GM-CSF plus G-CSF. A4 cell development to neutrophils is accompanied by major changes in the expression of protein kinase C (PKC) subspecies in these cells. The predominant subspecies present in multipotent A4 cells, as judged by direct chromatographic analysis, was the type III enzyme (alpha) subspecies, whereas in mature A4 cell neutrophils, the type II (betaI + betaII) enzymes were predominant. Phorbol esters added to immature A4 cells resulted in a proliferative response, but when added to postmitotic A4 cells resembling neutrophils they elicited a large increase in reactive oxygen intermediate production. This suggests that the type III (alpha) subspecies may mediate proliferative responses in stem cells, whilst the type II (betaI + betaII) enzymes are more important for the mature cell functions of postmitotic neutrophils. In cultures containing IL-3 (100 units/ml) both the type III, and also the type II subspecies were predominantly membrane-associated for prolonged periods (>24 hours). The addition of IL-3 (100 units/ml) to FDCP-Mix A4 cells starved of haemopoietic growth factors led to the rapid translocation of protein kinase C from the cytosol to the membrane; no such effect was observed with GM-CSF or 1 unit/ml IL-3. Under conditions where differentiation and development were induced (1 unit/ml of IL-3 plus GM-CSF and G-CSF), there was a redistribution of all PKC subspecies to the cytosol from the membrane. Thus, IL-3 preserves the multipotential nature of A4 cells and translocates PKC to the membrane. As GM-CSF cannot stimulate the translocation of protein kinase C, the differential biochemical and developmental effects of these growth factors on multipotent cells may in part be mediated by the activation of protein kinase C.