The proportion of proliferating cells in malignant non-Hodgkin's lymphomas (NHL) was determined in situ by immunostaining with the monoclonal antibody Ki-67, which reacts with a nuclear antigen that is present only in proliferating cells. A highly significant correlation could be demonstrated between the proportion of Ki-67 positive cells and the classification into high and low-grade NHL according to the Kiel classification. 93·8 per cent of high-grade and 88·5 per cent of low-grade malignancies were correctly allocated to these groups using the percentage of Ki-67 positive cells as discriminant parameter. On the basis of the medians, the degree of proliferation also paralleled the succession of entities within the Kiel classification. Within most of these different entities, however, the ranges of Ki-67 positive cells varied considerably, indicating that the growth fractions within these groups are rather heterogeneous. Thus it might be useful to determine the growth fraction of each individual case of NHL, because this might be of prognostic value.