Microglia were isolated from primary mixed brain cell culture of normal newborn mice and then cultivated. They were able to be maintained in vitro for 1-2 months, but incorporated little [3H]thymidine under normal culture conditions. When treated with the conditioned medium of L929 mouse fibroblast cells as a crude CSF-1 (mouse macrophage-colony stimulating factor) or purified CSF-1, microglia showed morphological changes and increased in both cell number and [3H]thymidine uptake. In addition, crude CSF-1 increased lysosomal enzyme activity and superoxide anion formation of microglia up to 2 and 3.8 fold as control value, respectively. These effects of CSF-1 were not observed in the purified astrocyte culture. Purified microglia had CSF-1 receptors which were recognized by the anti-CSF-1 receptor antibody that arose from a peptide of a product of proto-oncogene, c-fms. 12-O-Tetradecanoylphorbol-13-acetate (TPA) also increased microglia cell number and their biochemical activities, suggesting the possible involvement of protein kinase C activation. Protein kinase inhibitors, such as staurosporine or H-7, inhibited the effects of both CSF-1 and TPA. These results indicate that microglia may be regulated in its biochemical and proliferation activities by CSF-1 and that this may occur via activation of protein kinase C.