BALB/c mice injected with Moloney sarcoma virus developed within 5–12 days tumors at the site of inoculation which usually completely regressed by day 20–25. Maximal tumor size was reached around day 15. Sera from these animals were examined for complement‐dependent cytotoxic activity against Moloney lymphoma cells. The cytotoxic antibody response was biphasic. The first peak was reached at day 10 and extended to day 15 (peak tumor size). Twenty days after virus infection in the early regressor sera, antibody titres fell to normal serum values, followed by a progressive rise which persisted in the long‐term regressors during the entire observation period of 3 to 6 months. Active sera from days 10, 15, 30 and long‐term regressors were pooled separately and fractionated on Sephadex G‐200 into 19S and 7S fractions, and reconcentrated to the original volume. The fractions were then tested for cytotoxic activity. Both 19S and 7S fractions of all pools contained cytotoxic activity. Immunofluorescence analysis of these fractions showed the 7S fractions to be detectable by membrane immunofluorescence but not the 19S. Virus neutralizing activity was found in both 19S and 7S fractions with higher activity in the sera from regressor animals.