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An extract of the meningococcus antigens (MA) prepared from N. meningitidis was tested for an anti-tumor effect in rat and murine metastasizing tumor models. Effectiveness of MA in each model varied with dose and was manifested as significantly improved survival of the treated animals. Growth of the primary Fischer bladder carcinoma (FBCa) and metastases to lungs and lymph nodes were significantly inhibited in F344 rats treated weekly with 1 mg MA. Administration of MA at 100 micrograms per animal significantly prolonged survival of P815 mastocytoma-inoculated DBA/2 mice. Survival of C-26 colon adenocarcinoma-bearing Balb/c mice was significantly improved in animals that received weekly injections of 20 micrograms MA, without significant effect on the development of local tumor. The meningococcal antigens demonstrate strong mitogenic activity in B-cell-enriched murine spleen cultures. Thus the immunostimulatory activity of MA in experimental malignancy could involve, directly or indirectly, activation of B lymphocytes.