1. 1. Spectroscopic examination of T. cruzi microsomal fractions and intact epimastigotes have provided evidence for the presence of cytochrome P-450. the terminal oxidase of a reduced pyridine nucleotide-linked monooxygenase system involved in the metabolism of several lipid soluble compounds. 2. 2. The hemoprotein is consistently shown in cells previously treated with phenobarbital or in their microsomal fractions. 3. 3. Treatment with phenobarbital increases the levels of cytochrome P-450, which correlates well with increases in the N-demethylation of aminopyrine. 4. 4. The induction phenomenon is clearly observed in intact cells but is obscured in microsomal fractions apparently due to the presence of proteolytic enzymes that denature and/or degrade cytochrome P-450. Spectroscopic examination of mitochondria confirms the presence of two additional terminal oxidases, cytochrome oxidase and cytochrome o. 5. 5. No carbon monoxide-binding pigments are observed in the cytosolic fraction. 6. 6. It is suggested that the cytochrome-P-450-linked monooxygenase system in T. cruzi may be responsible for the resistance of the parasite to antimicrobial drugs. © 1976.