Birds exhibit physiological polyspermy, i.e. numerous spermatozoa enter thegerminal 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 cleavagestages. To establish a factor responsible for spermatozoal degeneration, thepresence of DNase activity was studied in vitro in extracts of Japanesequail oocytes using λ DNA/HindIII as a substrate. The experimentalconditions were designed to reveal the presence of either DNase I or DNaseII activities, separately. Degradation of the substrate DNA was evaluated byelectrophoresis on agarose gels stained with ethidium bromide. Highactivities of DNase I and DNase II were found in the germinal discs of thelargest vitellogenic oocytes. DNase I activity was estimated to be about3 × 10−3 Kunitz units and DNase II about 4 × 10−2 Kunitz units per germinaldisc. DNase I activity in an oocyte seems to increase during oogenesis sinceDNA degradation by the extracts from the germinal discs of the largestvitellogenic oocytes was much higher than by those from previtellogenic andsmall vitellogenic oocytes. The presence of high DNase I and II activitiesin the largest vitellogenic oocytes would point to their role in degradationof DNA from supernumerary spermatozoa entering the ovum during polyspermicfertilisation in birds. The enzymes could be a factor, or one of thefactors, in the late block to polyspermy in the cytoplasm of avian eggs. Itis suggested here that the DNase activities might also be responsible forpoor efficiency in obtaining transgenic birds by microinjection of exogenousDNA into the fertilised chick ovum.