Hepatic microsomal cytochrome P-450 from phenobarbital-pretreated rats is destroyed by propyne in a reduced nicotinamide adenine dinucleotide dependent process which also results in vivo in the accumulation of an abnormal green porphyrin. The green porphyrin has been identified by its electronic absorption, mass spectrometric, and nuclear magnetic resonance properties as the isomer of N-(2-oxopropyl)protoporphyrin IX in which the alkylated nitrogen is that of pyrrole ring A. Alkylation of the other nitrogens in the parent heme is quantitatively unimportant, although evidence for traces of the resulting adducts has been obtained. The green porphyrin exhibits a circular dichroism spectrum and is therefore the result of a chirally selective or specific interaction. The structure of the green porphyrin implicates a cytochrome P-450 destructive mechanism in which a species formed by catalytic oxidation of the acetylenic moiety reacts with the nitrogens of prosthetic heme. The possible nature of the reactive intermediate is discussed.