1. Because of the low concentration of cytochrome P-450 in avocado fruit, microsomal fractions were prepared using polyethylene glycol aggregation and low-speed centrifugation, thus avoiding the need for high-speed centrifugation of large volumes of post-mitochondrial supernatant. Recoveries of cytochrome P-450 by this means (0.29 nmol/g tissue) were similar to those after the usual high-speed centrifugation preparation (0.26 nmol/g). The cytochrome P-450 content of tulip bulb (0.30 nmol/g) was similar to that of avocado, but both plant tissues had much lower P-450 contents than did rat liver (13.0 nmol/g). 2. Spectral studies indicate that cytochrome P-450 of avocado mesocarp microsomal fraction binds fewer substrates than does the rat liver enzyme system. Type I binding spectra are given by fatty acids (C7-C14), aryl hydrocarbons (C7-C12), p-chloro-N-methylaniline and N,N-dimethylaniline. Type II binding is seen with inhibitors of mammalian cytochrome P-450 such as metyrapone, and with the imidazole antifungal agents such as clotrimazole. 3. These binding spectra provide a rapid method for identifying possible substrates and inhibitors of avocado cytochrome P-450, and also provide information concerning the nature of the active site of avocado cytochrome P-450. 4. Avocado cytochrome P-450 catalysed the N-demethylation of N,N-dimethylaniline (17.1 nmol/min per nmol P-450) and p-chloro-N-methylaniline (13.1 nmol/min per nmol P-450), and the hydroxylation of lauric (dodecanoic) acid (1.1 nmol/min per nmol P-450).