This study was designed to evaluate the effects of roxithromycin (RXM), a newly synthesized macrolide antibiotic on allergic responses in mice. RXM was orally administered into BALB/c mice once a day for 42 days in a single dose of 5 mg/kg body weight. Spleen cells (Sp C) collected from mice on day 7, 14, 28 and 42 post-RXM administration showed higher blastic activity of lymphocytes than those from control. The activity peaked on the 7th day, then gradually decreased, and returned to the control level by the 42nd day. Production of cytokines, IL-2 and IL-5, by Sp C in response to concanavalin A stimulation was also examined in the course of RXM administration. The capacity of Sp C to produce IL-2 was enhanced by oral administration of RXM for 28 days. However, a long-term (for 42 days) administration inhibited it. On the other hand, the capacity of of Sp C to produce IL-5 was strongly inhibited by oral administration of RXM; the titer of IL-5 was similar to that obtained in cultures of Sp C from control mice. These results strongly suggest that oral administration of RXM inhibits the function of Th2-type helper T lymphocytes and that a long-term administration of RXM may be beneficial in asthma and allergy.