The monoclonal antibody NK1 C3, synthesised by the Netherlands Cancer Institute, has been used to assess its value in the diagnosis of melanocytic lesions. The antigen recognised by this antibody is not denatured by formalin fixation, with the result that the antibody can be used for retrospective studies on conventionally processed material. Positive results were obtained in primary melanoma (18/18), secondary melanoma (21/21), junctional and compound naevi (32/32), intradermal naevi (9/12), congenital naevi (3/3), so called dysplastic naevi (13/13), blue naevi (5/5), and Spitz tumours (3/14). Non-melanocytic tumours were tested for comparison. The results showed relative but not complete specificity of the antibody for melanocytic tumours, with positive results only in breast and prostate tumours (2/6 and 2/5 respectively). Negative results were obtained with basal and squamous cell carcinoma, appendage tumours, neural tumours, and apudomas. The staining pattern of NK1 C3 was compared with that of antibodies to S100 protein and to neurone specific enolase. Compared with S100 protein NK1 C3 gave stronger staining of a higher percentage of cells in the 12 specimens in which a direct comparison was made. Antibody raised against neurone specific enolase in sheep gave very poor results with heavy background staining. We suggest that NK1 C3 is a useful addition to the battery of monoclonal antibodies of value to the diagnostic histopathologist.