Sea urchin spermatozoa were injected into mature mouse oocytes to determine whether they can activate mouse oocytes and, if so, how they behave within the oocyte cytoplasm of such a distant species. While injection of a single spermatozoon into each oocyte did not activate any of the oocytes, injection of 10 spermatozoa activated about 20%. Within the cytoplasm of unactivated oocytes, sperm heads commonly transformed into chromosome-like structures. When a single spermatozoon was injected, and oocytes were then activated by Sr2+, about 30% of the activated oocytes had both female (mouse) and male (sea urchin) pronuclei when examined 8 h after sperm injection. These results indicated that sperm-borne oocyte activating factor(s) and the cytoplasmic factors controlling the development of the sperm pronucleus are not strictly species-specific.