Twelve healthy subjects were placed on aspirin in a dosage of 600 mg five times daily for 14 days. The capability of their lymphocytes in culture to incorporate 3H-thymidine in response to phytohemagglutinin and allogeneic lymphocytes (one-way mixed lymphocyte culture) was measured before and after the course of aspirin. The results were compared with those obtained simultaneously from non-aspirin-treated control subjects. Statistical analysis showed no significant differences between lymphocyte response in the pretreatment and posttreatment periods or between aspirin-treated and non-aspirin-treated subjects. In contrast to previous reports of lymphocyte suppression resulting from addition of aspirin directly to lymphocyte culture and from orally administered aspirin, we conclude that comparable dose ranges in vivo do not suppress lymphocyte responsiveness to either phytohemagglutinin or allogeneic lymphocytes, at least within a 2-week period of treatment.