The intraperitoneal absorption properties of Ig purified from coho salmon serum were studied. In all experiments, aliquots of the radiolabeled immunoglobulin were compared in duplicate fish receiving the protein intraperitoneally and intraveneously. This experimental design allowed comparative absorption efficiency and kinetic measurements. The degree and kinetics of absorption were found to be temperature dependent. It was also determined that the monomeric subunit derived from the tetramer was absorbed with lower efficiency over relatively longer periods of time. Absorption results are discussed in terms of their phylogenetic significance.