The pharmacokinetics of a new macrolide antibiotic, josamycin base, has been assessed by a cross-over study on 15 healthy, young volunteers with 4 different dosages. The mean serum half-life was 1.5 h. The relationship between the log dose given orally and log peak serum concentration reached in these subjects was approximately linear. There was a wide variation in serum concentrations within the study group. The mean peak value was 1.3 µg/ml with 750 mg, 4.8 µg/ml with 1,250 mg and 8.1µg/ml with the 2,000-mg dose. It was found that even with the highest dose given, there was no danger of accumulation in subjects with a normal renal and hepatic status. When the response in serum concentrations and rapid excretion are held up against the sensitivity of Staphylococcus aureus, and compared to erythromycin it is concluded that josamycin base cannot be considered a first-hand antibiotic.