U937 cells are established human monoblast cell line originally isolated from a patient with diffuse histiocytic lymphoma. In the present study, superoxide anion (O2-) generation from U937 cells were measured by 2-methyl-6-[p-methoxyphenyl]-3,7- dihydroimidazo[1,2-a]pyrazin-3-one (MCLA)-dependent luminescence. Addition of 0.5 mumol/l MCLA and a stimulatory agent, such as N-formyl-methionyl-leucyl- phenylalanine (fMLP) and phorbol myristate acetate (PMA), to a suspension of the cells treated with gamma interferon (IFN gamma) caused a marked luminescence which was inhibited by 0.5 mumol/l superoxide dismutase (SOD). On the other hand, the cells which were not treated with IFN gamma caused no significant luminescence by the stimulation with either fMLP or PMA. The adherent IFN gamma-treated cells generated more O2- than the nonadherent IFN gamma-treated cells by the stimulation with fMLP. The maximal luminescence intensity from the adherent IFN gamma-treated U937 cells stimulated by fMLP was dependent upon the number of the cells. Azelastine (A-5610, CAS 58581-89-8) significantly inhibited the O2- generation from the adherent IFN gamma-treated U937 cells stimulated by fMLP or PMA in a dose-dependent manner.