Carcinoma in situ of the testis (CIS) is a precursor of invasive testicular germ cell tumours. The diagnosis of CIS is however often missed when conventional histological techniques are used. No specific immunological marker for CIS germ cells of the testis has been demonstrated previously. A novel monoclonal antibody, M2A, reacting with malignant germ cells of seminomas has recently been developed. Using the immunoperoxidase reaction on tissue sections, we tested the reactivity of M2A with CIS germ cells of the human adult testis. Positive reaction was found in 19 of 20 testicular specimens showing CIS, whereas no staining was found in 39 testicular biopsies without CIS. Thus, M2A may serve as a diagnostic marker in detection of CIS germ cells.