This study was designed to evaluate the effects of roxithromycin (RXM), a newly synthesized macrolide antibiotic, on cytokine production from mast cells. Mast cells, induced by long-term culture of spleen cells from BALB/c mice, were stimulated with 2.5 micrograms/ml concanavalin A in the presence or absence of various concentrations of RXM. The culture supernatants were obtained 24 h after stimulation. RXM caused a reduction in TNF-alpha levels in culture supernatants in a dose dependent manner and was first detected at a concentration of as little as 0.5 microgram/ml. Metabolized RXM (RU 39001, RU 44981, and RU45179) also suppressed TNF-alpha production in a dose dependent fashion with a minimum concentration of 0.5 microgram/ml. However, metabolized RXM, RU 28111, scarcely affected TNF-alpha production from cultured mast cells. These results strongly suggest that RXM inhibits mast cell function, especially inflammatory cytokine production and may result in favorable modification in inflammatory diseases.