Although cytochrome P-450 monoxygenases mediate critical reactions in plant microsomes, characterization of their activities has been difficult due to their inherent instability and the lack of a crossreacting P-450 antibody. We have surveyed the effects of protein stabilizing agents on t-cinnamic acid hydroxylase (t-CAH), a prominent microsomal P-450, and on total P-450 monoxygenase content. Trans-cinnamic acid is the most effective protecting agent for t-CAH activity. Leupeptin, a broad spectrum protease inhibitor, stabilizes t-CAH activity and increases the apparent P-450 content more than serine protease inhibitors such as phenylmethylsulfonyl fluoride. The combination of t-cinnamic acid and protease inhibitors increase the level of detectable t-CAH activity 4- to 14-fold over the levels detected by previously published procedures. In order to estimate the molecular weights and diversity of the plant P-450 monoxygenases in wounded pea epicotyls, we have prepared two polyclonal antibodies against the Pseudomonas putida camphor hydroxylase (P-450(cam)). One of the heterologous antibodies cross-reacts with constitutive microsomal polypeptides between 52 and 54 kilodaltons and several pea (Pisum sativum L.) mitochondrial proteins between 47 and 48 kilodaltons. The other polyclonal antibody cross-reacts strongly with two wound-induced polypeptides (65 and 47 kilodaltons) and weakly with one constitutive polypeptide (58 kilodaltons). We conclude that at least two subclasses of plant P-450 monoxygenases share common epitopes with the bacterial P-450 enzyme.