The pharmacokinetics of azithromycin, a new azalide antibiotic, were examined in man. Approximately 37% of a single oral dose of 500 mg was bioavailable and produced a peak serum concentration of 0.4 mg/l. Multiple dose regimens (two doses of 500 mg separated by 12 h and followed by 500 mg qds for five days, or two doses of 250 mg separated by 12 h and followed by 250 mg qds for nine days) produced only slight increases in peak serum concentrations. The serum protein binding of azithromycin declined from about 50% at 0.02 mg/l to 12% at 0.5 mg/l. Tissue concentrations of azithromycin were much higher than serum concentrations. After two 250 mg doses 12 h apart, peak azithromycin concentrations exceeded 3 mg/kg in prostate, tonsil and many other tissues. Concentrations in tissues declined with apparent half-lives of 2.3 days in prostate and 3.2 days in tonsil. The high tissue concentrations suggest that proposed standard dosage regimens of 500 mg qds on day 1 followed by 250 mg qds for four days, or three daily dosages of 500 mg, will produce tissue concentrations above 3 mg/kg in a variety of tissues. Since these tissue concentrations exceed the MICs of relevant pathogens, these dosage regimens should be effective against respiratory tract and soft-tissue infections. A single 1 g dose may be effective in the treatment of many sexually transmitted diseases.