Twenty-eight primary and 29 metastatic melanoma lesions and 18 pigmented nevi lesions were analyzed by using the immunoperoxidase reaction with anti-MART-1 and anti-gp100 monoclonal antibodies (mAbs). The MART-1 was expressed in 28, 29, and 18, and gp100 was expressed in 27, 28, and eight of these lesions, respectively. Intensity and percentage of stained cells with anti-MART-1 mAb were stronger and higher than those with anti-gp100 mAb. MART-1 was expressed homogeneously in primary melanoma and pigmented nevi, whereas it was heterogeneously expressed in metastatic melanoma lesions. The level of expression of MART-1 in primary melanoma lesions did not correlate with any clinicopathologic parameters. These results suggest that anti-MART-1 mAb is a useful tool for immunohistochemical analysis of melanocytic lesions and also is useful for patients' selection and monitoring of antigen-loss variants in clinical trials with the MART-1-based immunotherapy.