We have previously shown that irradiated LSTRA cells (LX) were immunogenic and could prolong survival of mice bearing LSTRA tumors. This study demonstrated that addition of Corynebacterium parvum to the LX dramatically improved the strength of tumor immunity. In pretreatment-challenge experiments, C. parvum augmented the immunogenicity of 10(6) LX given intradermally, with an optimum dose of 0.14-1.4 microgram C. parvum per mouse. In therapy experiments (intraperitoneal vaccine treatment after tumor cell challenge), the therapeutic effect of 10(7) LX was improved by admixture of C. parvum, leading to a larger number of cured mice, permitting treatment of a larger challenge inoculum, and allowing later initiation of treatment than was possible with LX alone. Optimal dose of C. parvum for therapy was 1,400 microgram per mouse. Presensitization to C. parvum or use of repeated vaccine injections did not further improve the therapeutic effect. Cure of tumor-bearing mice by the mixed vaccine was tumor-specific. These results suggest that C. parvum is a potent adjuvant for use in active, tumor-specific immunotherapy.