Antitumor activity of marine bacteria, V. anguillarum P-B-1, against Ehrlich carcinoma cells in ddY mice was investigated. Ehrlich carcinoma cells were inoculated i.p. into mice and V. anguillarum (1.0 mg/mouse) was administered i.p. twice before and 4 times after the tumor inoculation. Groups of mice administered V. anguillarum 80-90%; their mean survival time was 54.0-56.0 days (range, 24.0-60 days) at day 60 after inoculation of 104-106 tumor cells against the mean survival of 16.4-22.3 days (ranged, 7-29) in the control group. When 2 .times. 106 tumor cells were inoculated s.c. mixed with 1.0 mg of V. anguillarum, the bacterial cells markedly suppressed the growth of tumor at the injection site; tumor did not grow in 30% of recipient mice and the inhibition rate of grown tumor in the rest of recipients was 69%. The consistently demonstrable antitumor activity of V. anguillarum was reduced by pretreatment of mice with immunosuppressants such as anti-thymocyte serum, hydrocortisone or x-ray irradiation. The antitumor activity of V. anguillarum apparently is mediated by immune response.