Components of mycobacterial cell wall(s) (CW) attached to oil droplets were evaluated for their ability 1) to inhibit the growth of line-10 tumor transplants in the skin of syngeneic guinea pigs when inoculated together with 106 tumor cells (suppression experiments) and 2) to regress established 7-day-old intradermal tumors and eradicate microscopic lymph node metastases upon injection into the tumors (regression experiments). CW and cell-wall skeleton (CWS) preparations from Mycobacterium phlei, a fast-growing saprophyte of group IV of the atypical mycobacteria, suppressed tumor growth in essentially all animals when 37.5-μg doses were administered; at a dose of 300 μg, they cured 50–60% of the animals in regression tests. The addition of 300 μg of a purified trehalose mycolate, isolated from M. tuberculosis strain Aoyama B, to 300 μg M. phlei CW or CWS preparations significantly increased their tumor regressive potency to provide cure rates to about 90%. Because M. phlei can be propagated more readily, it can be used advantageously in place of BCG to prepare stable, nonliving immunologic adjuvants of defined composition and consistently high potency to meet the need for standardized agents in experimental cancer immunotherapy. It also may be of value in treating patients with minimal residual malignant disease.