A method for purification of rainbow trout, Oncorhynchus mykiss, serum immunoglobulin (Ig) that relies on specific retention of rainbow trout Ig in the presence of other serum proteins, by an ion-exchange chromatography matrix (ABx resin) previously developed for the isolation of mammalian Ig is described. This protocol allows for rapid and substantial purification of total Ig including antigen-specific antibodies from the serum of rainbow trout. The procedure does not utilize antibody-derivatized matrices for immunoabsorption so it is suited to situations where fish Ig-specific antisera or monoclonal antibody is unavailable. Furthermore, total Ig can be isolated from pre-immune sera since no antigen-derivatized affinity matrices are used, eliminating the requirement for lengthy immunization regimens. With minor modification, ABx-chromatography can be adapted for the isolation of Ig from other species of fish. © 1995 Taylor & Francis Group, LLC.