We studied the pharmacokinetic interaction between cyclosporin (CYA) and erythromycin in normal subjects. Plasma CYA concentrations were measured by high performance liquid chromatography (h.p.l.c.) and radioimmunoassay (RIA) and estimates of metabolite formation were obtained from inter-assay differences between these measurements. Erythromycin significantly increased the maximum concentration and the area under concentration-time curve. Time to maximum concentration and apparent oral clearance of CYA were significantly decreased. The half-life, however, was not altered. Significant reductions in the proportion of apparent metabolite were observed at times of maximum CYA concentrations but not at later time periods (12 and 24 h). The mechanism of the drug interaction appears to be decreased hepatic first-pass metabolism but an effect on CYA absorption cannot be excluded. These results on normal subjects confirm that patients administered CYA and erythromycin risk CYA toxicity. However, the risk can be reduced by dose reduction based on more frequent CYA monitoring or by using a different antibiotic.