Effects of azelastine (AZ) on human peripheral blood leukocytes (PBL) in culture were examined. Addition of 10.0 micrograms/ml of AZ resulted in a marked inhibition of PBL blastic activity, but lower concentrations (1.0 and 0.5 microgram/ml) had no demonstrable suppressive effects on the activation. Interleukin (IL)-2, IL-3 and IL-4 production from PBL in response to Concanavalin A stimulation was also strongly suppressed when the cells were cultured in the presence of AZ. This suppression was observed even when lower concentrations (1.0 and 0.5 microgram/ml) of AZ were added to cell cultures.