Rats have been treated with several derivatives of the erythromycin, erythralosamine or oleandomycin series, in order to compare their ability to induce cytochrome P-450 and to form stable 456 nm-absorbing cytochrome P-450 metabolite complexes. The data obtained confirm that the cytochromes P-450 induced in rats by various macrolides are similar to that induced by pregnenolone 16 alpha-carbonitrile: the cytochrome P-450 IIIA1 isozyme. It showed that: (i) formation of a stable inhibitory 456 nm-absorbing cytochrome P-450 complex is not a prerequisite for cytochrome P-450 induction but enhances induction by stabilization of the IIIA isozyme. Therefore, the best inducers lead also to the maximal in vivo amounts of cytochrome P-450 metabolite complex (except for 2'MBEM); (ii) affinity for cytochrome P-450 IIIA1 is not directly involved for induction; and (iii) hydrophobicity favors induction and formation of complexes. Structural factors are also involved.