A cytolytic factor partially purified from eggs of the sea hare Aplysia kurodai was examined for cytolytic activity against various target cells. All kinds of tumor cells tested were lysed in vitro by the cytolytic factor in the range of 10-100 ng protein/ml. In contrast, normal spleen cells were lysed by 10 micrograms/ml of this factor and red cells were not lysed even at this higher concentration. Tumor lysis was time-dependent and was complete within 10 hr. This factor inhibited DNA and RNA syntheses of tumor cells but not protein synthesis. The cytolytic activity was lost on heat-treatment (60 degrees) and at pH 2, and was partially inhibited by treatment with 8M urea and at pH 12, but the factor was resistant to treatments with trypsin, periodate and 2-mercaptoethanol. Neutralizing activity was observed in vivo on pretreatment of MM46 and L1210 cells with the factor. This cytolytic factor also inhibited the growth of solid-type MH134 tumor and ascitic-type MM46 tumor. These results indicate that Aplysia eggs contain a novel cytolytic factor that lyses tumor cells in vitro and inhibits tumor growth in vivo.