We studied cytokeratin (CK) expression immunohistochemically in 64 seminomas using a panel of commercially available antikeratin antibodies and tested for association of CK expression with patient age, tumor size, stage, and outcome. Seventeen embryonal carcinomas were compared with seminoma. CK7, CAM 5.2, AEI/AEIII, and wide-spectrum screening keratin (WSK) were positive in 41%, 30%, 36%, and 36% of the seminomas, respectively. CK20 and high-molecular-weight keratin (HMWK) were negative in all cases. CD30, placental alkaline phosphatase (PLAP), and epithelial membrane antigen (EMA) were positive in 6%, 100%, and 2% of cases, respectively. There were no differences in patient age, stage, tumor size, or outcome between CK-positive and CK-negative seminomas. CK7, CAM 5.2, AEI/AEIII, and WSK were positive in 100%, 88%, 94%, and 88% of embryonal carcinomas, respectively. CK20 and HMWK were negative in all cases. CD30, EMA, and PLAP were positive in 100%, 12%, and 76%, respectively. CKs are present in seminoma, and their presence is not associated with a difference in patient age, stage, or outcome. In cases such as small needle biopsy specimens, CK and CD30 stains may be useful in separating seminoma from embryonal carcinoma.