Mouse monoclonal antibody M2A is specific for seminomas and their malignant carcinoma in situ (CIS) germ cell precursors. M2A has been used for the histopathological diagnosis of testicular and extratesticular seminomas. Targeting of99m Tc-M2A to subcutaneous human tumor xenografts in nude mice allowed imaging of these tumors by gamma-scintigraphy, suggesting a potential of this reagent for surveillance of patients following orchiectomy for a primary testicular seminoma.