Sparfloxacin disk susceptibility test criteria for the NCCLS and ICS/DIN methods were determined by testing 400 bacterial wild-type isolates. Disks containing 5 micrograms of the drug could be used satisfactorily in both procedures. The following interpretive zone size breakpoints for the NCCLS and ICS/DIN methods were proposed: less than or equal to 18 and 20 mm respectively for resistance (MIC greater than 1 mg/l), and greater than or equal to 23 and 25 mm respectively for susceptibility (MIC less than or equal to 0.5 mg/l). These criteria were based on preliminary maximum serum concentrations of approximately 1.5 mg/l after a single oral dose of 400 mg of the drug. Regression equations for both methods correlating MICs and zone sizes based on test results for 361 organisms are presented. The equations allow calculation of breakpoints for higher or lower serum peak levels. Sparfloxacin was two to four times more active than ciprofloxacin against gram-positive cocci, and showed equal activity against gram-negative rods. Zone diameter quality control values for Escherichia coli ATCC 25922 were 26-30 mm (NCCLS) and 30-33 mm (ICS/DIN), and for Staphylococcus aureus ATCC 29213 27-28 mm (NCCLS) and 29-31 mm (ICS/DIN).