SummaryConcentrations of erythromycin in seminal plasma and in blood plasma were investigated in 11 healthy volunteers after a 1 g oral dose, and in 15 patients with sub-clinical prostatitis treated with 500 mg 8-hourly for 3 to 5 days. In the volunteers, peak concentrations of the antibiotic in blood plasma were noted 3 to 4 hours after drug intake and 4 to 6 hours after in seminal plasma. In the patients, similar mean values and ranges to those in the volunteers were found in seminal plasma. Analysis of the various fractions of split ejaculates from the volunteers revealed similar secretory patterns for zinc and for erythromycin, indicating that the antibiotic was mainly secreted in the seminal plasma by the prostate.