In photopheresis peripheral leukocytes are exposed to photoactivated 8-methoxypsoralen in an extra-corporeal flow system. The present study was performed to characterize the effect of photopheresis upon the function of peripheral circulating neutrophils. Superoxide generating capacity and chemotactic ability of peripheral circulating neutrophils were tested in seven patients immediately before and after treatment with photopheresis. Superoxide generation stimulated by phorbol-12-myristate-13-acetate and N-formyl-methionyl-leucyl-phenylalanine (fMLP) and fMLP-induced chemotaxis were found not to be influenced by photopheresis. We conclude that circulating neutrophils obtained from patients treated with photopheresis possess normal superoxide-generating capacity and chemotactic ability.