Polymorphonuclear leukocytes (PMNs) actively internalize ciprofloxacin, a capability that can enhance killing of intracellular bacteria and facilitate delivery of the antimicrobial agent to infection sites by migrating PMNs. In this study we investigated mechanisms for up-regulation of this process. Activation with N-formyl-methionyl-leucyl-phenylalanine (fMLP; 100 nM) enhanced PMN ciprofloxacin uptake by 50% (P < 0.05). Phorbol myristate acetate (PMA; > or = 10 nM) enhanced uptake by at least 36-fold, mainly by stimulating an increase in the Vmax of the ciprofloxacin transporter. This effect of PMA was inhibited by antagonists of protein kinase C (H7 and chelerythrine) and the mitogen-activated protein kinase cascade downstream (PD 098059). Under resting and PMA-activated conditions, ciprofloxacin uptake by immature human promyelocytic leukemia (HL-60) cells was much lower than in PMNs. However, when HL-60 cells were induced to mature into PMN-like cells, their ciprofloxacin uptake activity increased markedly. These findings implicate a role for protein kinase C in up-regulation of the ciprofloxacin transporter and suggest that myeloid cells acquire an enhanced ability to take up ciprofloxacin as they mature to end-stage PMNs.