Effects of the methanol extraction residue (MER) fraction of tubercle bacilli (BCG) on the generation of cytotoxic lymphoid cells were studied in vitro with the use of unidirectional mixed lymphocyte-tumor cell cultures. These cultures consisted of splenocytes of lymph node cells from normal donor C57BL/6, BALB/c, and strain A mice and mitomycin C-inactivated leukemia cells of both syngeneic and allogeneic origin. Addition of small amounts of MER (0.2-5 microgram/ml) to the cultures potentiated appreciably the elicitation of cytotoxic reactivity (as measured by the 51Cr-release assay) of the sensitized cells, whereas higher quantities (10-40 microgram/ml) had a strong suppressive effect. MER also induced some cytotoxic capacity in normal murine and human lymphoid cells not exposed to specific tumor cell stimulation. The stimulatory and suppressive effects were noted only when MER was present during the initial 24-48 hours of the 6-day culture. With the nylon wool fractionation technique, it was apparent that MER affected primarily the nonadherent cell population. MER could also prevent the generation of nonspecific suppressor cells by splenocytes maintained for 3-6 days in tissue culture.