AbstractIn order to derive information about possible mechanisms by which the sperm head is converted into the male pronucleus during fertilization in the rabbit, unfertilized egg homogenate was assayed for two enzyme activities. Protamine was extracted from rabbit sperm, purified, and labelled with [14C] in an in vitro reaction and used as a probe to assay for a protein kinase which could transfer [32P]PO4from [γ‐32P]ATP onto the substrate. A kinase with a pH optimum of approximately 8.0 to 8.5 is described. Assays for the enzyme glutathione reductase were performed using homogenates from eggs or embryos at three early stages of development. Results suggest that oocytes can oxidize 2.58 × 10−6μmol NADPH per minute per oocyte, unfertilized eggs 5.16 × 10−7μmol NADPH per minute per ovum, and 20‐ to 24‐hour postcoitus fertilized eggs 2.30 × 10−6μmol NADPH per minute per ovum. The relevance of these observations to male pronuclear form