One hundred and six melanocytic lesions were studied to determine the value of the so called Ag NOR technique in differentiating dysplastic naevi, Spitz naevi and spindle cell naevi of Reed from malignant melanoma and from 'banal' compound or intradermal naevi. In 79 cases Ag NOR counts were possible. The banal naevi (36) had a mean count of 1.54, (SD 0.3), and the unequivocally malignant superficial spreading melanomas (13), lentigo maligna melanomas (4) and secondary melanomas (4) had a mean count overall of 3.9 (SD 1.59). For dysplastic naevi (16) the mean Ag NOR count was 1.63 (SD 0.36) and for Spitz and spindle cell naevi (in toto 6) the figure was 1.72 (SD 0.55). The difference in Ag NOR counts between all types of naevi and all types of melanoma was highly statistically significant, but there was no difference between banal naevi and dysplastic, Spitz, and spindle cell naevi. Correlation of Ag NOR counts between three independent observers was good. This technique may, therefore, be a useful adjunct in separating true melanoma from borderline melanocytic naevi.