Isolated microglia underwent in vitro morphological transformation to ameboid, rod-shaped and ramified forms in response to several cytokines, factors from astrocytes, or factors which affect macrophage function. These morphological transformations were reversible. Examination of the enzymatic activity, superoxide anion formation and proliferative activity of these transformed microglia indicated that the ameboid forms were activated, rod-shaped microglia were proliferating, and the ramified forms were resting microglia.