Tween 80 (polyoxyethylenesorbitan monooleate) was found to have anticancer activity in the SJL/J tumor system. Investigations to evaluate variables of importance in treatment revealed a loss of inhibitory influence when Tween 80 was administered by the subcutaneous rather than the intraperitoneal route of injection. Age at the onset of treatment was also a factor with significant reductions in incidences of reticulum cell sarcoma when treatment was initiated at 30 or at 40 weeks, but not at 20 weeks of age. Optimal inhibitory effects were achieved with concentrations of 5% Tween 80 (0.2 ml/mouse) or 10% Tween 80 (0.1 ml/mouse). No differences in inhibitory influence on SJL/J tumorigenesis were observed with variation in the frequency of treatment (biweekly, weekly or twice weekly injections).