A methanol-insoluble residue (MER) of phenol-killed attenuated tubercle bacilli (BCG), which has been reported previously to be capable of evoking heightened resistance to infection with antigenically unrelated microorganisms, was found to affect as well the resistance of highly inbred mice against tumor isografts. In most instances, the MER evoked heightened resistance against the tumor implants, but heightened susceptibility was the effect induced against two of the tumors tested, and no effect was elicited against one neoplasm. It is suggested that the heightened susceptibility occasionally produced by pretreatment with MER may also be of immunological nature, i.e. immunological enhancement. Treatment with MER was more effective when administered some time before tumor challenge than when given simultaneously with, or after, tumor implantation. The protective effects manifested against some tumors were of a high order, a significant number of animals rejecting the neoplastic implants, and were displayed even when several months elapsed between treatment and challenge. Living BCG and intact phenol-killed bacilli also evoked heightened resistance against some of the tumors tested, and in one experiment living BCG proved effective whereas MER did not. On the whole, however, MER was the most active (and least toxic, as shown previously) of the several tubercle bacillus preparations tested. MER elicited heightened reactivity against first transplant generation tumors as well as against tumors maintained for considerable periods of time by repeated animal passage, and against spontaneously arising as well as against induced neoplasms. The experimental parameters necessary to demonstrate maximal effects varied somewhat from tumor to tumor. In general, however, single intraperitoneal injections of small quantities of MER, of the order of 0.25 to 1.0 mg, afforded the best protection.