The MIB1 monoclonal antibody which is used as a marker of cell proliferation was studied by immunohistochemistry on formalin-fixed and paraffin embedded biopsy specimens of lymph nodes in 40 high- and 46 lowgrade cases of non-Hodgkin's lymphomas (NHL) classified according to the Kiel classification. All cases were found to display nuclear MIB1 staining. A statistically significant difference (P < 0.005) was found between high- and low grade NHLs and this indicates that the high- grade NHL display a higher proliferation rate than low grade. In addition, remarkable variations in MIB1 expression were found among individual cases of the same histological group. These data may suggest that MIB1 index can help in the individual approach of the proliferation rate of each tumour and this may be an important parameter in association with clinical and other laboratory parameters for predicting the biological behaviour of non-Hodgkin's lymphomas.