Immunoperoxidase staining using five antisera to human carcinoembryonic antigen (CEA), including a mouse monoclonal antibody, was performed to investigate the expression of CEA reactivity in ten cases of granular cell myoblastoma. The granular cells were negative with four of the antisera although control sections of CEA producing colon carcinoma were positive. The single positive antiserum gave intense granular cytoplasmic staining of all tumour cells in the ten specimens studied. This reactivity was abolished after absorption of the antiserum with a perchloric acid extract of human lung to remove cross‐reacting antibodies against non‐specific cross‐reacting antigen (NCA); a procedure which did not affect the staining of colon carcinoma specimens. The results indicate that the granular cells do not contain CEA but express a related antigen and that care in the choice of primary antiserum is important if the immunocytochemical detection of this antigen is to be used as a diagnosti