A syngeneic murine lymphoid leukemia, designated MCAS-l0, was utilized as a model system to test chemoimmunostimulation therapy. Treatment of leukemic mice with 1,3-bis(2-chloroethyl)-I-nitrosourea (30 mg/kg) suppressed the disease for approximately 9–10 days before relapse and eventual death of 65% of the drug-treated animals. A significant number of long-term survivors (80–100%) were obtained in groups of animals that received combined drug and BCG treatment, in contrast to groups of mice treated with BCG or drug alone. As few as 80 viable BCG organisms afforded as good a protection to mice as 8 × 107 living organisms. Similar results were obtained when chemoimmunostimulation therapy was administered against an advanced stage of the leukemia. The protective effect by BeG appeared to be independent of route of administration. When given during the induced remission period, 2 defined chemicals pyran copolymer and tilorone hydrochloride as well as Corynebacterlum granulosum, a killed nonspecific immune stimulator, were found very effective against this leukemia. The implications of these results to cancer therapy are discussed.