A b s t r a c t Inhibin is a heterodimeric glycoprotein originally detected in gonadal tissues. One report described inhibin immunopositivity in 17 ofA recent report described inhibin immunopositivity in 17 of 19 hepatocellular carcinomas (HCCs) and the hepatocytes of the surrounding nonneoplastic parenchyma. 9Based on these results, the authors proposed that immunostaining for inhibin may be valuable in the differentiation of HCC from other neoplasms involving the liver. The reported immunohistochemical method that used the avidin-biotin complex did not describe a step to block endogenous biotin. Since liver tissue often contains high levels of biotin, and if not blocked, endogenous biotin may result in false-positive immunostaining, 10 " we wondered whether the reported immunopositivity of HCC and the surrounding hepatocytes for inhibin represented a falsepositive result due to unblocked endogenous biotin in the neoplastic and nonneoplastic hepatocytes. By using a similar microwave antigen retrieval technique and the same specificity, titer, and clonal source of primary antibody as the aforementioned study, immunostaining for inhibin was performed with and without an endogenous biotin blocking step on 23 cases of HCC and the surrounding cirrhotic