We have examined the generation of superoxide by human monocytes isolated from peripheral blood cultured in the presence of recombinant human interleukin-3 in comparison to tumor necrosis factor-alpha and interferon-gamma. The rate of superoxide production of unstimulated and stimulated monocytes [by formyl-methionyl-leucyl-phenylalanine (0.1 microM) and by phorbol-myristate-acetate (2 ng/ml and 200 ng/ml)] decreased during the culture period in the absence of interleukin-3, whereas cells treated with interleukin-3 maintained or surpassed their initial superoxide-producing capacity. An increase of phorbol-myristate-acetate- and formyl-methionyl-leucyl-phenylalanine-stimulated superoxide production of monocytes cultured with interleukin-3 compared to control cells was detected first after 24 h of monocyte culture. It was maximal after 96 h of monocyte culture. At this time the stimulated superoxide production of monocytes cultured in the presence of interleukin-3 surpassed that of interferon-gamma and tumor necrosis factor-alpha treated cells. Suboptimal concentrations of the stimulus phorbol-myristate-acetate (2 ng/ml) resulted in higher priming than 200 ng/ml phorbol-myristate-acetate. A dose dependence of the effect of interleukin-3 on the superoxide production was observed. Our results demonstrate that interleukin-3 primes cultured human peripheral blood monocytes for enhanced stimulated respiratory burst activity to a higher extent than interferon-gamma and tumor necrosis factor-alpha.