In the present study, the hepatic microsomal and peroxisomal bifunctional trans-2-enoyl CoA hydratases were isolated and purified from rats treated with 2% di-(2-ethylhexyl)phthalate for 8 days. These two enzymes (microsomal and peroxisomal) were purified with the identical purification procedures and had identical molecular masses of 76 kDa. A single band was observed on an electrophoretic gel of an equimixture of the two proteins. Both preparations had identical pI's of 8.6 and pH optima of 6.0 for the dehydrogenase (reductase) and 7.5 for the hydratase activity. Two-dimensional gel analysis of an equimixture of the two preparations showed only one band. Ouchterlony double-diffusion analysis showed that an antibody raised against the purified microsomal enzyme interacted at a point with the peroxisomal enzyme, indicating immunologic identity. Western blot analysis demonstrated that the antibody formed a single band with total microsomal and peroxisomal fractions. The antibody inhibited the enzymatic activities of both preparations in a similar manner. Interestingly, the antibody had a markedly greater inhibitory effect on the reductase activity of the two enzyme preparations, and a much less inhibitory effect on the hydratase activity, suggesting that the antigenic determinants reside at or near the catalytic site of the reductase portion of the protein. These results suggest that the microsomal and peroxisomal bifunctional proteins are identical.