A relatively crude enzyme preparation derived from the subcuticular epithelium of earthworms catalyzed the formation of 4-hydroxyproline from prolyl residues in unhydroxylated natural collagens and in several synthetic collagen-like polypeptides. The specificity of hydroxylation differed from that of all vertebrate polyl hydroxylases in that (Gly-Pro-Ala)n was a much better substrate than (Gly-Ala-Pro)n. In contrast, however, only the so-called Y position proline (Gly-X-Y) was hydroxylated in Gly-Pro-Pro sequences derived either from natural collagen or from synthetic polypeptides; specificity of hydroxylation for the latter sequence is identical with that of the vertebrate enzymes. Little or no formation of 3-hydroxyproline could be demonstrated in preparations of the enzyme active as a 4-hydroxylase. In contrast with an earlier report from another laboratory, using a crude extract of earthworm body wall, we were unable to demonstrate either significant 3-hydroxyproline formation or efficient 4-hydroxylation of X position prolyl residues in synthetic polypeptides with the internal sequence Gly-Pro-Pro.