Aplysianin-A, an antibacterial and antineoplastic factor in the albumen gland of the sea hare Aplysia kurodai, was isolated. It had a molecular weight of approximately 320 kD and consisted of subunits with a molecular weight of 85 kD. It contained 9.8% neutral sugar. Aplysianin A showed 50% inhibition of Bacillus subtilis growth at a concentration of 4 microgram protein/ml and 50% lysis of murine MM46 tumor cells at 14 ng protein/ml. A partial identity of antigenic specificity of the purified specimen with an antineoplastic factor from Aplysia eggs was observed in immunodiffusion tests.