The relationship between labeling indices of Ki-67 reactive antigen expressed by cycling cells and BrdUrd incorporated into S-phase cells was investigated in 20 patients with malignant tumors. Both of the labeling indices varied greatly from patient to patient; the labeling index of Ki-67 ranged from 37.5% to 1.9% with an average value of 16%, and the BrdUrd labeling index ranged from 23.4% to 1.6% with an average of 9.3%. The Ki-67 labeling index was higher than the BrdUrd labeling index. In general, the values of the Ki-67 labeling index were parallel to those of the BrdUrd labeling index, and the relation Y = 1.59X + 1.15 (r = 0.89) was obtained. In human solid tumors, therefore, the growth fraction can be easily estimated from the S-phase fraction size, and vice versa.