Abstract We report that rabbit antibodies against goldfish immunoglobulins (C‐Ig) bind goldfish erythrocytes (C‐RBC) in agglutination and immunofluorescence assays. These reactions can be inhibited by absorption of rabbit antiserum with goldfish spleen leukocytes and either C‐Ig or their large fragments (> 40 kd M.W.) obtained after peptic digestion, but not with small fragments (< 20 kd). Trypsin treatment of C‐RBC enhances the agglutination titre of the rabbit antiserum even after 24 hrs incubation of trypsinized C‐RBC in tissue culture medium. The present data show that C‐RBC have membrane molecules which cross‐react with C‐Ig and therefore could be relevant in furthering our understanding of the biology of the nucleated RBC in lower vertebrates.