1. By several criteria, macrolide antibiotics constitute a new class of nitrogenous cytochrome P-450 metabolic-intermediate complex-forming compounds. 2. Macrolide antibiotic metabolic-intermediate complexes are only formed in livers induced with phenobarbital or with the macrolide antibiotics themselves. The extent of metabolic-intermediate complex formation in microsomes from phenobarbital-induced rats is lower than that seen for members of the amphetamine and SKF 525-A classes of compounds. 3. Cytochrome P-450 induced by macrolide antibiotics, of which troleandomycin is the most potent, is extensively sequestered as a metabolic intermediate complex in vivo. 4. Cytochrome P-450 induced by troleandomycin differs, using several criteria, from those induced by phenobarbital, beta-naphthoflavone or SKF 525-A, and those present in uninduced rats.