Some studies have suggested that the addition of ciprofloxacin to in vitro cultures of mitogen-stimulated lymphocytes exerts inhibitory effects on cell cycle progression and immunoglobulin (Ig) secretion. We tested the effects of this drug on some immunity parameters in BALB/c mice. Mice treated intraperitoneally with ciprofloxacin (10 mg/kg of body weight per day) for 3 consecutive days and immunized with sheep erythrocytes 24 h after the last injection showed significant suppression of hemolytic IgG-forming cells, whereas the response of IgM-forming cells remained unchanged. When treatment lasted 7 days the response of antibodyforming cells was not modified. When the 3-day treatment was started at 24 h after immunization with sheep erythrocytes, the response of IgM-forming cells was increased, whereas the response of IgG-forming cells was suppressed. Delayed-type hypersensitivity to sheep erythrocytes was significatively suppressed in animals that received the 3-day treatment with ciprofloxacin and were immunized subcutaneously 24 h after the last injection. In vitro proliferation of lymphocytes from ciprofloxacin-treated mice in response to either lipopolysaccharide or concanavalin A was also suppressed. Leukopenia and an increase in the level of granulocytemacrophage colony-forming cells in bone marrow were also observed in ciprofloxacin-treated mice. These results, together with those from other reports, suggest that modification of the biological responses by ciprofloxacin is a complex phenomenon that may be influenced by several factors.The knowledge of possible influences of antibiotics on the immune response seems to be of great importance for the clinical approach to the process of therapy. This is specially interesting when antibiotics are given to immunosuppressed patients. The capacity to modify the immune response to unrelated antigens has been observed with numerous antibiotics, including -lactams (8, 19), aminoglycosides (19), tetracyclines (10), rifamycins (2), and others.The fluoroquinolones are antimicrobial agents with a broad antimicrobial spectrum (21). The primary target of fluoroquinolones is the bacterial DNA gyrase (topoisomerase II) (21). However, some inhibitory effects of these agents on the eucaryotic enzymes involved with DNA replication have been observed (9). A number of quinolones have been shown to possess genotoxicity in vitro, and this has been considered a possible explanation for some of the abnormal eucaryotic cellular responses obtained after treatment with these agents (9). Some of these effects have been observed on leukocytes. It has been found that ciprofloxacin penetrates leukocytes and reaches intracellular levels with bactericidal activity (4,20). The incorporation of [ 3 H]thymidine into T lymphocytes incubated with phytohemagglutinin was increased in the presence of fluoroquinolones, but the progression of mitogen-stimulated lymphocytes through the S and G 2 /M stages of the cell cycle and the secretion of immunoglobulins (Ig's) by pokeweed mitogen-stimulated B ...