The rates of cis to trans interconversion of Glt-(Ala)n-Pro-Phe-4-nitroanilides (n = 1-3) were estimated by means of a two-step process with chymotrypsin as the trans-substrate cleaving activity. By the aid of this system, pig kidney and several other tissues contained demonstrable catalytic activity against the cis to trans interconversion of the proline containing peptides. The active protein fraction was purified 38-fold from pig kidney cortex by ammonium sulfate precipitation and a series of column chromatographic techniques. Activity was detected against the cis to trans interconversion of Glt-Ala-Ala-Pro-Phe-4-nitroanilide to a different extent. No activity was found with Phe-Pro-4-nitroanilide. With respect to the substrate specificity, this enzyme must be classified as a peptidyl-prolyl cis-trans-isomerase. The enzyme was strongly inactivated by p-chloromercuribenzoate, sodium dodecylsulfate, Hg2+- and Cu2+-ions, but was not inhibited by metal chelators, diisopropylphosphorofluoridate and chlorotosylamidophenylbutane. The activity is abolished by incubation with trypsin. The enzyme is heat sensitive at 50 degrees C. The results presented in this paper suggest a new type of enzymes, characterized by catalytic activity against conformational interconversions. The possibility of the location of the enzyme on ribosomal particles is discussed.