Disaggregated neopallial cells from newborn C3H/HeJ mice were cloned in Grenier hybridoma tissue culture dishes, and culture wells that contained only one cell were marked. After 8-10 days of culturing, the cultures were fixed and double immunolabeled for microglia with Mac-1 antibody and for astroglia with antibody to GFAP. Each marked well containing a clone was identified as either a microglia, astroglia, mixed microglia-astroglia, or an unlabeled clone. The effect of LM cell line conditioned medium (LM-CM), which contains colony-stimulating factor-1, on the development of mixed microglia-astroglia clones was determined. Formation of mixed clones was dose dependent (P < 0.0001). We concluded that microglia and astroglia have a common progenitor cell and that the development of mixed clones is LM-CM dependent.