The cellular reactivities with the monoclonal antibodies OKT9 and Ki 67 have been demonstrated to be closely related to proliferation in various malignant neoplasms. In this study a total of 25 melanocytic skin tumors was examined immunohistochemically with both antibodies and the results were evaluated semiquantitatively for OKT9 and quantitatively for Ki 67 by stereological methods. All cases of primary and metastatic malignant melanoma expressed a strong stainability for OKT9, whereas benign melanocytic nevi were almost completely negative. Our results with the monoclonal antibody Ki 67 revealed highly significant differences in the numerical density of Ki-67-positive cells between metastatic malignant melanoma (number of positive cells: 47.0 +/- 9.2 X 10(3)/mm3), primary malignant melanoma (6.3 +/- 1.9 X 10(3)/mm3) and benign melanocytic nevi (2.2 +/- 0.7 X 10(3)/mm3). Correlation analysis between mean percentage of OKT9-positive cells and numerical density of Ki-67-positive cells revealed a significant correlation of both parameters (r = 0.58; p less than or equal to 0.05), indicating a positive relationship of OKT9 and Ki 67 expression. Especially in primary malignant melanoma, however, the amount of OKT9-positive cells considerably exceeds that of Ki-67-positive cells. The monoclonal antibodies OKT9 and Ki 67 reflect 'proliferative activity' in melanocytic skin tumors, as both are expressed in significantly higher amounts in primary and metastatic malignant melanomas. The combined application of these antibodies in cutaneous melanocytic lesions might be of diagnostic and prognostic value.