A radioactive soluble model protein for studies of protein digestion, absorption and amino acid (AA) metabolism in larval fish was successfully produced in Atlantic salmon by oral administration of uniformly [U]- 14C-labelled amino acids followed by blood sample withdrawal (48 h post-administration) and purification. The salmon serum protein ( 14C-SSP) was characterised in terms of the protein composition and specific activity of its amino acids. Most radioactivity was found in the three most abundant serum proteins, which had apparent molecular weights of 65, 75 and 120 kDa, respectively, of which labelling was found in all the amino acid residues of the SSP that were analysed. The digestibility of the 14C-SSP was tested by in vivo tube feeding using early stages of Atlantic halibut and was found to be more efficiently digested and utilised than the 14C-methylated bovine serum albumin (14C-BSA) that has been used in previous studies. This supports the notion that proteins labelled by 14C-methylation are not suitable as model proteins in metabolic studies due to modification of their 14C-methylated lysine residues. Further studies on the 14C-SSP demonstrated a digestibility of 59±13% in juvenile halibut, while at the pre-metamorphic stage, it was only 25±13%. This supports the hypothesis that there is a significant improvement in the ability to digest and utilise dietary proteins as the digestive system becomes fully developed, including a functional (acid-producing) stomach. © 2004 Elsevier B.V. All rights reserved.