Surgical specimens of 29 human thyroid masses, both benign and malignant, were examined by means of a novel monoclonal antibody immunoreactive to statin, which is expressed only in quiescent G0 cells. The nuclei of normal thyroid follicle tissue together with nodular goitres and follicular adenomata had similar labelling indices of 96 +/- 2.67, 95 +/- 2.43 and 94 +/- 1.98 respectively. By contrast the labelling indices of papillary and undifferentiated thyroid malignancies were 82 +/- 3.05 and 15.2, respectively. These results indicate that normal thyroid tissues as well as benign thyroid tumors have similar non-proliferative activities. The differentiated papillary cancers have a smaller non-cycling compartment, the smallest being present in the most biologically aggressive undifferentiated thyroid cancer. The immunohistological evaluation of statin in the nuclei of human thyroid malignancies correlates with their biological behaviour in an inverse relationship.