The intravenous and intraperitoneal administration of ciprofloxacin in very high doses (2 x 1 mg/d up to 2 x 2 mg/d) can reduce the bacterial load of mice experimentally infected with the intracellular bacterium, Listeria monocytogenes. The T-cell response generated during the treated infection is affected in much the same way as it is during antibiotic treatment with ampicillin, i.e. the protective immunity established directly correlates with the number of bacteria present during an extended period of time during the primary infection. Although an additional anti-proliferative effect of ciprofloxacin on expanding T-cells as evidenced in in vitro experiments cannot be excluded, our data in summary favour the view that in vivo this effect is at most of minor importance.