The production and secretion of plasminogen in cultured rat brain microglia was investigated. Urokinase-dependent caseinolytic activity was detected by zymography in microglial conditioned medium with a molecular weight of about 90 kDa. The 90-kDa protein was also detected by Western blotting with anti-rat plasminogen antiserum in the non-reducing condition. Immunoprecipitation with plasminogen antiserum following [35S]methionine labelling revealed that the plasminogen detected in microglial conditioned medium is synthesized in microglia. The amount of plasminogen in the conditioned medium was increased by stimulation with lipopolysaccharide. These results show that cultured microglia produce plasminogen and secrete it into the culture medium.