A review of European data published before July 1986 and data from the authors' laboratory showed very similar activity of ciprofloxacin against gram-positive bacteria in all studies. Mean minimal inhibitory concentrations against 50 percent (MIC50) and 90 percent (MIC90) of staphylococcal strains were 0.32 and 0.59 mg/liter, respectively. The drug was equally active against staphylococcal strains resistant and susceptible to methicillin and/or gentamicin. The range of MIC50 and MIC90 values for Streptococcus faecalis and Streptococcus faecium was between 0.25 and 1 mg/liter and 1 to 8 mg/liter, respectively. Ciprofloxacin inhibited group A and group B streptococci at concentrations of 0.5 to 2 mg/liter. Group C and G strains were less susceptible (MICs, 2 to 16 mg/liter). Penicillin-resistant and penicillin-susceptible pneumococci and the Viridans streptococci were inhibited by 0.5 to 4 mg/liter. MIC50 and MIC90 values of ciprofloxacin against Corynebacterium, including group JK, and anaerobic gram-positive cocci were 1 and 8 mg/liter, respectively, and against anaerobic gram-positive rods, were 2 and 16 mg/liter, respectively. Listeria monocytogenes strains were inhibited by 0.12 to 2 mg/liter. Ciprofloxacin showed bactericidal activity against staphylococci and streptococci in minimal bactericidal concentration tests and in killing kinetic studies. The in vitro activity of the drug was influenced neither by the method employed, by the medium used, by the pH of the medium, nor by the size of the inoculum. Resistance to ciprofloxacin developed in staphylococci in a step-wise manner. First-step mutants usually showed a fourfold to eightfold decrease in susceptibility. Contrary to the situation in gram-negative organisms, resistant mutants did not show reduced growth.