In addition to their antibacterial properties, certain antibiotics are also biologic response modifiers. The fluorinated 4-quinolone, ciprofloxacin, for example, up-regulates IL-2 and IFN-gamma production in PBLs stimulated in vitro. In the present study, ciprofloxacin was shown to increase the levels of mRNA for IL-1 alpha, IL-2 and its receptor, IFN-gamma, IL-3, IL-4, granulocyte-macrophage/CSF, TNF-alpha, and lymphotoxin. As investigated with different T cell mitogens and alloantigens, the up-regulation of IL-2 production by ciprofloxacin was found to be independent of the mode of stimulation. Analysis of transcription activity showed that ciprofloxacin enhances IL-2 gene induction. The concentrations of nuclear factor of activated T cells (NF-AT-1) and AP-1 were also found to be increased by ciprofloxacin. Thus, ciprofloxacin interferes with a regulative pathway common to several cytokines.