The melanoma-associated antigen (MAGE) gene family of cancer-testis antigens is expressed in certain malignant neoplasms and the testis, but not in other healthy tissues. The aim of this study was to determine the usefulness of immunohistochemical staining with the 57B anti-MAGE-A4 mouse monoclonal antibody (MAb) in testicular biopsy specimens from patients with nonobstructive azoospermia and obstructive azoospermia (OA). Fifty-four cases of Sertoli cell only (SCO), 30 cases of spermatocytic arrest, 15 cases of hypospermatogenesis, and 10 testicular biopsy specimens with OA (normal spermatogenesis) were evaluated. Immunohistochemistry was performed using the 57B MAb, which primarily recognizes the MAGE-A4 antigen in paraffinized tissues. The cells were quantitated by a computerized image analysis system. Testicular biopsy specimens with normal spermatogenesis exhibited strong nuclear and cytoplasmic MAGE-A4 staining of spermatogonia and weak staining of spermatocytes, but not spermatids or Sertoli or Leydig cells. No staining was detected in SCO cases. In five cases of SCO with focal spermatogenesis, spermatogonial cells that were initially missed by hematoxylin and eosin staining were detected by MAGE-A4 immunohistochemistry. Immunostaining with the 57B MAb greatly enhanced identification of spermatogonia in cases of spermatocytic arrest and hypospermatogenesis. The number of MAGE-A4-positive spermatogonia was significantly decreased in hypospermatogenesis, as opposed to the OA group (12.1 +/- 4.3 and 30.3 +/- 10.0, respectively). The number of MAGE-A4-positive primary spermatocytes was significantly increased in early maturation arrest, as compared with the OA group (48.2 +/- 10.8 and 16.9 +/- 9.8, respectively). The 57B anti-MAGE-A4 MAb is a useful marker for the detection and quantitation of spermatogonial germ cells. It also facilitates automated image analysis and provides greater accuracy in the histopathologic evaluation of testicular biopsy specimens.