The cytokine interleukin (IL)-12 plays a bridging role between innate and adaptive immunity. Here, we demonstrate that treatment of neutrophils with IL-12 leads to a transient increase in intracellular-free calcium [Ca(+)(+)](i) levels, which is necessary for the production of reactive oxygen metabolites (ROM). This production is associated with the activation and nuclear translocation of the transcription factor nuclear factor (NF)-kappaB and is inhibited in the presence of the intracellular calcium chelator 1,2-bis(O-amminophenoxy) ethane-N,N-N',N'-tetraacetic acid-acetoxymethyl ester and the ROM production inhibitor diphenyl iodonium. We show that IL-12 causes a significant increase in total mRNA levels, which appear dependent on the generated ROM. In addition IL-12 induces the de novo synthesis and production of IL-8 and tumor necrosis factor alpha (TNF-alpha) in a calcium- and ROM-dependent manner. Our data demonstrate a direct role for IL-12 in the activation of human neutrophils and suggest a ROM-dependent interplay between IL-12-induced [Ca(+)(+)](i) transient and the release of IL-8 and TNF-alpha through NF-kappaB activation.