Several derivatives of the erythromycin, erythralosamine and oleandomycin series have been prepared. Their abilities to bind to rat liver microsomal cytochrome P-450 and to lead to the formation of stable 456 nm absorbing cytochrome P-450-metabolite complexes after their oxidative microsomal metabolism in vitro have been compared. The obtained data confirmed that cytochrome P-450 induced in rats either by macrolides or by 16 alpha-pregnenolone carbonitrile were the major isozymes involved in the binding of macrolides to liver microsomes and in metabolite-complex formation. They showed that (i) hydrophobicity was in general a beneficial factor for these two properties, (ii) the presence of a bulky substituent in position 3 of erythromycin dramatically decreased their affinity for these isozymes, and (iii) the simultaneous presence of bulky substituents in position 2' and 3 prevented iron-metabolite complex formation. These results led to the selection of two compounds, erythralosamine-2'-benzoate and erythralosamine-2',3-diacetate, which exhibited a particularly high affinity for macrolide inducible cytochrome P-450 and were very good precursors of cytochrome P-450-iron-metabolite complex formation.