Birds exhibit physiological polyspermy, i.e. numerous spermatozoa enter the germinal disc of an oocyte and form pronuclei during fertilisation. However, only one of them unites with the female pronucleus to form a zygote nucleus; the supernumerary spermatozoal nuclei degenerate at the early cleavage stages. To establish a factor responsible for spermatozoal degeneration, the presence of DNase activity was studied in vitro in extracts of Japanese quail oocytes using λ DNA/HindIII as a substrate. The experimental conditions were designed to reveal the presence of either DNase I or DNase II activities, separately. Degradation of the substrate DNA was evaluated by electrophoresis on agarose gels stained with ethidium bromide. High activities of DNase I and DNase II were found in the germinal discs of the largest vitellogenic oocytes. DNase I activity was estimated to be about 3 × 10−3 Kunitz units and DNase II about 4 × 10−2 Kunitz units per germinal disc. DNase I activity in an oocyte seems to increase during oogenesis since DNA degradation by the extracts from the germinal discs of the largest vitellogenic oocytes was much higher than by those from previtellogenic and small vitellogenic oocytes. The presence of high DNase I and II activities in the largest vitellogenic oocytes would point to their role in degradation of DNA from supernumerary spermatozoa entering the ovum during polyspermic fertilisation in birds. The enzymes could be a factor, or one of the factors, in the late block to polyspermy in the cytoplasm of avian eggs. It is suggested here that the DNase activities might also be responsible for poor efficiency in obtaining transgenic birds by microinjection of exogenous DNA into the fertilised chick ovum.