The tracer added to the protein and peptide test solutions used in the current study (DOI http://dx.doi.org/10.1007/s002270100675; PROT and PEPT diets) consisted of [methyl 14C]-methylated bovine serum albumin (BSA). After the study was performed, we were made aware that the use of methylated BSA as the test protein may have underestimated the proteolytic capacity (Dr. K. Dabrowski, Ohio State University, USA, personal communication). A study conducted by Murakami and Etlinger (1987) demonstrated that in vitro hydrolysis rates of methylated BSA by two mammalian cytosolic (reticulocyte and muscle) proteases were lower than those of unmodified BSA. The reduced proteolytic activity in the study of Murakami and Etlinger (1987) was attributed to a blockage of amino groups of lysine residues that make them less available for conjugation reactions. Whether this also applies to intestinal proteolytic enzymes and assimilation of lysine in fishes remains to be shown. However, a lower digestive absorption rate of protein than of free amino acids in post-larval Atlantic halibut is in line with previous data for juveniles and adults of other teleosts (Atlantic cod, Berge et al. 1994; Atlantic salmon, Espe et al. 1993; rainbow trout, Yamada et al. 1981) and also of other vertebrates, including man (Metges et al. 2000). Still, the results for assimilation of protein and peptides in Atlantic halibut post-larvae need verification with the use of non-methylated compounds.