Roach (Rutilus rutilus) were irradiated with a single dose of ultraviolet B (UVB) radiation (0.4 J/cm2) in order to study the effects of UVB on the nonspecific immune defense mechanisms of fish. Neutrophils and macrophages were isolated from the head kidney of fish on days 1-14 postirradiation. Both random and directed migration of neutrophils, studied by migration under agarose assay, were suppressed on day 1 after UVB irradiation. The respiratory burst of phorbol 12-myristate 13-acetate-stimulated neutrophils and macrophages was also suppressed at days 1 and 2 after UVB irradiation. The suppression of migration and respiratory burst were restored or the responses were even enhanced later, but on the other hand spontaneous cytotoxicity of neutrophils toward 51chromium-labeled K562 target cells stayed suppressed throughout the 14 day follow-up. This study indicates that UVB radiation has the potential to suppress the functioning of phagocytes and to compromise the immune system of fish.