Arca subcrenata Lischke is a marine traditional Chinese medicine. The study investigated the antitumor effects of P2, a polypeptide fraction from A. subcrenata, and its toxicity in vitro and in vivo. The results showed that P2 could inhibit the proliferation of seven tumor cell lines, especially in HeLa and HT-29 cell lines. The IC₅₀ values were 11.43 μg/mL for HeLa and 13.00 μg/mL for HT-29 treated by P2 for 48 h. P2 had little cytotoxicity on normal liver cells (L-02). The maximum tolerated dose (MTD) of P2 on KM mice was 1000 mg/kg by i.p. or i.v. The tumor growth inhibitory ratios of P2 were 26.4%, 41.4% and 46.4% for H-22, and 34.0%, 45.8% and 60.1% for S-180 tumor-bearing mice. The results demonstrated that P2 might be a potential antitumor agent with high efficiency in dose-dependent and time-dependent manners and low toxicity.