The effects of three quinoline derivatives--pefloxacin, ciprofloxacin and ofloxacin--were investigated in mitogen-stimulated human peripheral blood mononuclear leucocytes (MNL). At concentrations of 50 mg/l or more, pefloxacin, ciprofloxacin or ofloxacin significantly inhibited MNL proliferation in response to phytohaemagglutinin. This inhibition was more marked with ciprofloxacin than pefloxacin or ofloxacin. To determine the possible mechanism(s) involved in the inhibition of MNL proliferation following exposure to pefloxacin, ciprofloxacin or ofloxacin, we assessed (1) interleukin-1 (IL-1) activity in supernatants from monocytes treated with the quinolones and (2) the effects of 2-mercaptoethanol (2-ME) a thiol compound which acts as an antioxidant agent and the effect of indomethacin (INDO) an inhibitor of prostaglandin E2 synthesis. 2-ME and INDO did not prevent the decrease in the proliferation. IL-1 activity was shown to be decreased for the same range of antibiotic concentrations as observed for the inhibition of MNL proliferation. Cellular viability of the MNL or monocytes was not modified by any of the quinolones at the concentrations tested. Taken together, these results suggest that pefloxacin, ciprofloxacin and ofloxacin act as immunomodulators. The mechanism involved with the cascade of events that leads to the lymphocyte proliferation and the clinical relevance need further investigation.