In studies of the antineoplastic factors from marine invertebrates, we previously found seven glycoproteins in the marine mollusks, Aplysia kurodai and Dolabella auricularia. We designated these glycoproteins aplysianins and dolabellanins which have a molecular weight of 60,000-320,000 dalton. In this work, we sought the antineoplastic factor from the body wall of Dolabella auricularia, and found the eighth factor which we designated dolabellanin B1. The factor was purified by gel filtration and two types of high performance liquid chromatography. Partially purified dolabellanin B1 was a single polypeptide of 42 kDa. Dolabellanin B1 was heat labile, showing appreciable loss of activity after heating at 55 o C for 10 min. The factor was stable at neutral pH (6-8). These results were the same as other dolabellanins. Tumor lysis by this factor begins within 30 min, but the red blood cells are not lysed. These results suggest that dolabellanin B1 is not a detergent. Dolabellanin B1 lysed MM46 tumor cells at 10 ng/mL, but had no antibacterial or antifungal activity. Six out of seven of the other dolabellanins specifically lysed tumor cells after 10 hours, completely inhibiting the syntheses of macromolecules such as DNA, RNA and protein before causing cytolysis. The cytolysis of tumor cells was completely inhibited by N-acetylneuraminic acid. However, N-acetylneuraminic acid did not inhibit tumor lysis induced by dolabellanin B1. Dolabellanin B1 lysed tumor cells within a short time. These results suggest that Dolabellanin B1 is a new type of antineoplastic protein different from other dolabellanins.