Post-translational hydroxylation occurs in three mammalian ribosomal proteins, uS12, uL2, and uL15, which are located in the small (S) and large (L) subunits of the ribosome near the most important decoding and peptidyltransferase functional centers. We have used cell cultures, which produce protein uL15 labeled with the 3xFLAG epitope at the C-terminus (uL15^3xFLAG) or mutant forms of uL15^3xFLAG that contain His39Ala, His39Thr, or His40Ala substitutions, to examine the role of hydroxylated His39 of uL15 in maintaining the translational activity of ribosomes. It has been found that exogenous uL15^3xFLAG is able to functionally replace endogenous uL15 in HEK293 cells transfected with an appropriate DNA construct. However, the translational activity of ribosomes decreases by about 35% in cells that produce the above mutant forms of uL15^3xFLAG compared with that in cells that produce nonmutated uL15^3xFLAG. Analysis of the structural model of the human ribosome has allowed us to assume that the hydroxyl group in His39 is involved in the local stabilization of the ribosome structure through the formation of a hydrogen bond between this group and the nitrogen atom of the His40 imidazole ring. Given that His39 is located near the E site of the ribosome, we believe that this stabilization of the ribosome structure ensures the maintenance of its translational activity.