Anti-sense mos oligonucleotides have been shown to block steps required for meiotic maturation of oocytes. In Xenopus oocytes, the block prevents germinal vesicle breakdown (GVBD), an early step in oocyte maturation. In mice the block induced by anti-sense mos occurs at a later step in oocyte maturation concerned with the first polar body emission. Here, we show that mouse oocyte maturation is blocked by introduction of anti-mos antibodies into immature functional oocytes. Antibodies that inhibit the mos kinase blocked GVBD. In contrast, anti-mos antibodies that permit the mos kinase to function do not block GVBD, but interfere with polar body formation. Antibodies pre-reacted with excess cognate peptide had no observable effects on maturation. These results indicate that the c-mos kinase function is required for activation of maturation-promoting factor (MPF) during meiosis. These findings are also consistent with our previous studies which show that the mos kinase directly phosphorylates cyclin B2, a component of MPF (Roy et al., 1990).