Bioactive polymers were sought in marine arthropoda and a novel cytolytic factor was found in a hermit crab, Clibanarius longitarsus. The partially purified factor showed activity in fractions corresponding to a molecular weight of about 10 kilodaltons on a Sephadex G-75 column. This cytolytic factor was halfmaximally active for tumor cells at 0.13-0.66μg/ml and for normal cells at 1.9-82μg/ml. Tumor lysis by the factor was time dependent and was complete within 12h. This bioactive polymer was labile on heating, at low and high pH.