The relation between the [3H]-thymidine labelling index (3H-Thy LI), which evaluates the S-phase cells, and the monoclonal antibody Ki-67 (MoAb Ki-67), which recognizes a nuclear antigen expressed during the cell cycle, was defined in 35 non-Hodgkin's lymphomas (NHL). Significantly higher median values of [3H]-Thy LI and Ki-67-positive cells were observed for high-grade than for low-grade malignancy tumours according to the Kiel classification, but with wide and overlapping values for the two morphologic subgroups. A significant correlation was observed between [3H]-Thy LI and Ki-67-positive cells (P less than 0.0001; r = 0.62). However, the ratio between the two cell kinetic variables on individual tumours was not constant. It sharply increased with decreasing [3H]-Thy LI values and was much higher in low-grade NHL than in high-grade NHL. Follow-up studies are needed to establish the role of the tumour growth fraction as evaluated by MoAb Ki-67 as a prognostic indicator in NHL.