Blood serum collected from female hamsters on each day of the oestrous cycle was assayed for its ability to induce the acrosome reaction. The maximum activity was found in sera obtained from females around the time of ovulation, and the minimum on Day 2. Ovariectomy did not affect the activity of the serum but oestrogen treatment significantly increased it. Blood serum from progesterone-treated and pregnant females significantly depressed the incidence of the acrosome reaction. Progesterone counteracted the effect of oestrogen with regard to the incidence of the acrosome reaction. The counteracting effect was maximal when both hormones were administered at the same time.
The possible rôle played by the hormones in this phenomenon is discussed.