The interaction between cyclosporin A (CsA) and the macrolide antibiotic, erythromycin, has been studied in freshly isolated rabbit hepatocytes and in rabbit liver microsomal fractions. In hepatocytes, CsA was rapidly accumulated inside the cells and metabolized to its different groups of derivatives (mono- and/or dihydroxylated and/or N-demethylated metabolites) [Fabre, Bertault-Peres, Fabre, Maurel, Just, and Cano: Drug Metab. Dispos. 15, 384 (1987)]. In the presence of erythromycin in the extracellular compartment, CsA metabolism was inhibited in a concentration-dependent manner. However, erythromycin did not affect intracellular CsA accumulation and binding of CsA to its intracellular protein binding site(s). Since CsA was specifically metabolized by the cytochrome P-450 LM3c isozyme [Bertault-Peres, Bonfils, Fabre, Just, Cano, and Maurel: Drug Metab. Dispos. 15, 391 (1987)], we further studied the effect of erythromycin on CsA metabolism by liver microsomal fractions. In the presence of erythromycin, CsA metabolism was also decreased. Lineweaver-Burk analysis of erythromycin-CsA interaction demonstrated that erythromycin was a competitive inhibitor (Ki = 156 microM) of CsA metabolism (Km = 0.43 microM; Vmax = 4.8 nmol/min). In agreement with these data, CsA inhibited (i) erythromycin N-demethylation to a large extent and (ii) the appearance of the erythromycin-cytochrome P-450 LM3c complex. We could conclude that the interaction between CsA and erythromycin most likely results from the fact that both drugs are extensively metabolized by the same cytochrome P-450 form: P-450 LM3c or P-450 III A4 according to the new nomenclature.(ABSTRACT TRUNCATED AT 250 WORDS)