Exposure to tubal fluid in vitro causes loosening of the corona radiata of recently ovulated ova, pretreated with hyaluronidase. Incubation of ova in 0·05 ml of fluid results in denudation of ova to the zona pellucida if mechanical agitation is provided by shaking with admixed sand. Control ova in Krebs-Ringer phosphate solution, Gey's solution, hyaluronidase or rabbit serum are unaltered under similar conditions. Both oestrous and postovulatory fluid are effective. The responsible factor is dialysable, heat-stable and survives lyophilization.