Pig plasma amine oxidase is irreversibly inactivated by phenylhydrazine. The stoichiometry of this inactivation was determined by monitoring the loss of catalytic activity, the formation of a new visible spectral band, changes in the circular dichroic spectrum and by equilibrium binding studies. In all cases, only 1 mol of phenylhydrazine reacted with the dimeric pig plasma amine oxidase; further additions of phenylhydrazine had no effect. Pretreatment of the enzyme with phenylhydrazine inhibited the binding of amine substrate. The phenylhydrazine-enzyme complex was stable under various experimental conditions for at least 72 h. Circular dichroic spectra revealed the conformation of the phenylhydrazine-treated enzyme to be altered in the region around prosthetic groups and indicated some changes about the aromatic amino acids. No major conformational changes were detected by this technique. Isoelectric focusing experiments exposed no differences in the band pattern or isoelectric point between the untreated and phenylhydrazine-treated enzymes.