It is known that cultured human epidermal keratinocytes produce C3, but it remains to be elucidated how such production is controlled. This study is designed to investigate what kind of cytokines are able to enhance C3 production by cultured human keratinocytes and what kind of signal transductional pathways are related. Methods: Human keratinocytes were cultured in serum free medium (KGM) in the absence or presence of a single cytokine (ILs, IFNs, TNFs, or TGFs) for 48 hs. Culture supernatants were harvested and assayed for C3 by a sandwich ELISA method. Changes of C3 mRNA were investigated with a Northern blot analysis technique using C3 DNA probe made by PCR. To examine signal transductional pathways, keratinocytes were pretreated with either PKC or PTK inhibitor. Results: The results showed that cultured human keratinocytes produce small amount of C3 constitutively and that the inflammatory cytokines enhanced remarkably with a prior increase in mRNA for C3, suggesting transcriptional control of C3 production by these cytokines. Pretreatment of keratinocytes with either PKC and PTK inhibitor caused abrogation of the enhancing effect of these cytokines. However, preincubation with an inhibitor of src-type PTK (herbimycin A) enhanced further C3 production. Conclusion: Present study indicates that C3 production was enhanced by the inflammatory cytokines and that C3 production was regulated by PKC- and PTK-dependent pathway(s), which is also negatively regulated by src-type PTK. We think that inflammatory reactions produce a micro-environment in the lesional skin where the complement activation take place effectively via enhancement of C3 production by the inflammatory cytokines.