1. The influence of spiramycin coadministration on cyclosporin pharmacokinetics was studied in five renal transplant patients. The plasma concentrations of cyclosporin were measured both by non-specific radioimmunoassay (RIA) and high-performance liquid chromatography (h.p.l.c.). 2. The kinetics of cyclosporin were followed before treatment, and after 1 day and then 2 weeks of oral treatment with spiramycin (3 X 10(6) iu, twice daily). The main pharmacokinetic parameters (the area under the plasma drug concentration-time curve, the maximum plasma drug concentration and the time to reach it) obtained both by RIA and h.p.l.c. were not modified by spiramycin cotreatment after 1 day, nor after 2 weeks of spiramycin administration. Therefore, the pharmacokinetics of cyclosporin (parent drug and parent drug plus metabolites) are not influenced by the coadministration of spiramycin macrolide at therapeutic dosage. 3. Spiramycin may be preferable to other macrolide antibiotics known to interact with cyclosporin such as erythromycin or josamycin.