Antisera to human beta 2 microglobulin (beta 2M) detected a plasma membrane molecule on goldfish (Carassius auratus) cells in immunofluorescence. A goldfish molecule detected by radioimmunoassay (RIA) co-eluted with human beta 2M on gel filtration. By affinity chromatography on immobilized antibody to human beta 2M, a molecule was purified (from extracts of goldfish) that showed, on SDS-polyacrylamide gel electrophoresis, a mobility similar to that of human beta 2M (apparent Mr 12,800 +/- 500).