The use of ECG admixed with tumor cells as a vaccine to induce systemic tumor immunity for therapy of subclinical (micrometastatic) disease was evaluated. In several experiments inbred strain 2 guinea pigs were given i.v. injections of 104, 105 or 106 syngeneic L10 hepatocarcinoma cells, and initial vaccinations were administered 1 or 4 days after tumor inoculation. Variables in vaccine preparation, such as ratio of viable BCG organisms to tumor cells, procedures for freezing the tumor cells, X-ray treatment of tumor cells and vaccination regimen were evaluated. Under defined conditions nontumorigenic vaccines of BCG and tumor cells can cure the majority of animals of otherwise lethal visceral micrometastases.