Metabolic activation of inherently nonprotein-reactive compounds (prohaptens) in the skin can lead to development of contact allergy, a chronic skin disease. The prohapten hypothesis has existed for more than 20 years; yet, detailed knowledge regarding the mechanisms of activation as well as what structural moieties can be transformed to protein-reactive sensitizers is still limited. Today, the consideration of cutaneous bioactivation is important when developing nonanimal-based assays for prediction of contact allergenic activity, as only methods that include skin metabolism are able to detect prohaptens as sensitizers. We have studied the mechanism of activation of the prohapten carvoxime (1), a strongly sensitizing but in itself poorly protein-reactive alpha,beta-unsaturated oxime. alpha,beta-Unsaturated oximes represent a novel class of prohaptens, which previously have never been investigated for potential metabolic activation. To identify reactive metabolites formed from 1, liver microsomal incubations in the presence of glutathione were carried out. Putative reactive metabolites were synthesized, and their allergenic activity, chemical reactivity toward nucleophiles, and ability to elicit a contact allergenic response in animals induced with 1 were assessed. We found that 1 is metabolically activated by epoxidation of the allylic carbon-carbon double bond. The alpha,beta-epoxy oxime metabolites were found to be sensitizers of extreme potency in the local lymph node assay and highly reactive toward nucleophilic amino acids and a model peptide. One of the two diastereomeric epoxy metabolites also elicited an allergic reaction in mice sensitized to 1, in the mouse ear swelling test. Furthermore, this study presents strong indications that the basis of the high reactivity and sensitizing capacity observed for the alpha,beta-unsaturated oximes is related to their ability to form highly reactive nitroso intermediates by tautomerization. To our knowledge, the formation of nitrosoalkenes by oxidative metabolism of alpha,beta-unsaturated oximes has not been shown so far.