Immunohistochemical analysis employing a monoclonal antibody nm23-H1 (the antibody against nm-23 protein) was performed on archival material, consisting of 12 cases of follicular thyroid carcinoma (FTC), 57 cases of follicular thyroid adenoma (FTA) and 17 cases of nodular goiter (NG). Both cytoplasmic and nuclear immunoreactions for nm-23H1 were observed in cells of FTCs, FTAs and NGs. In oxyphilic adenomas cytoplasmic staining was observed. Eleven (91.7%) cases of FTC, 55 (98.2%) cases of FTA and 14 (82.4%) cases of NG were found to be positive for nm23-H1 protein. There were no statistically significant differences in the mean percentage values of immunopositive cells between carcinomas and adenomas. A significant increase in the number of cases with high percentage (more than 50) of positive cells was found in both carcinomas (FTCs) and adenomas (FTAs)--mainly microfollicular ones, in comparison with nodular goiter. It can be concluded that highly positive immunoreaction for the nm23-H1 protein in the cells of carcinomas (FTCs) and microfollicular adenomas indicates for a high proliferation rate of these tumors.