Proliferating cultured P388 cells exhibited a greater degree of sensitivity to adriamycin than did proliferating cultured L1210 cells, although both leukemia cell populations had approximately the same doubling time. The rate of reduction in viability when cultured L1210 cell populations were exposed to adriamycin (0.0625-2.0 microgram/ml, concentrations that are comparable to tissue drug levels during therapy) was concentration-dependent. Therefore, the results indicated a possible therapeutic advantage to be gained by an increase in drug concentrations (within the limits of acceptable host toxicity) at the target cell site.