Maturation-promoting factor and a homolog of fission yeast cdc2+ gene product (p34cdc2) were investigated during the final 24 hr of maturation of quail oocytes. Kinase activity of p34cdc2 in the oocyte germinal disk (GD) increased 15 times at maturation. Two bands, at 32 and 34 kDa, were detected in immature oocytes by immunoblotting of SDS-PAGE with anti-p34cdc2 monoclonal antibody. A new band, which is close to the 32-kDa band but with a slightly faster mobility, appeared during maturation. No p34cdc2 could be detected outside the GD. Microinjection of GD extract from mature oocytes caused maturation of Xenopus oocytes.