We determined the optimal conditions for conducting experiments with the solid and ascites sublines of the 13762 rat mammary adenocarcinoma and examined the response of the tumor growth rate to BCG administered in admixture with tumor cells or separately at a remote site. Versene dissociation of the 13762 solid tumor produced better growth rates than did pronase-DNase, but the former decreased cell viability and yields. A dose of 10(6) or 10(5) tumor cells produced 100% growth by the sc and iv routes. Both sublines grew slower but produced metastases slightly sooner in the intradermal than in the sc site. The frequency of axillary lymph node metastases from the sc site increased as a function of the duration of the time interval between tumor implantation and surgical excision. Both solid and ascites tumors were weakly immunogenic. Administration of BCG in a split adjuvant protocol did not improve tumor immunity. Admixture of tumor cells with BCG suppressed tumor growth but when given at a remote site, BCG was ineffective. We concluded that the 13762 rat mammary adenocarcinoma is a useful system for BCG immunotherapy.