The correlation between AgNOR (silver stained nucleolar organizer region) protein quantity and the percentage of S-phase cells was determined in 45 human tumours of different origin (28 from endometrium, 8 from cervix, 4 from ovary, 3 from larynx, 1 from rhinopharynx and 1 from tongue). AgNOR protein quantity was evaluated by image analysis on histological sections stained by the one-step silver staining method. S-phase cells were detected in histological sections with anti-bromo-deoxyuridine (BrdU) antibodies after in vivo BrdU infusion. The relationship between AgNOR protein and BrdU labelling index (LI) values was tested by linear regression analysis. A highly significant association of the two parameters was found (r = 0.78; p < 0.001), independent of the tumour type. DNA content of the 45 tumours was also determined by flow cytometry, and the DNA quantity was correlated with the kinetics data. Euploid tumours had a mean AgNOR protein area of 3.33 +/- 1.34 microns2 and a mean BrdU-LI of 10.78 +/- 8.94%, while in aneuploid tumours the mean AgNOR protein area was 4.78 +/- 1.78 microns2 and the mean BrdU-LI 17.37 +/- 8.79%. Both AgNOR protein quantity and BrdU-LI were significantly higher in aneuploid than in euploid tumours (Student-t Test: p < 0.05 and p < 0.02 for AgNOR and BrdU values, respectively). However, despite this significant difference, a relevant overlap of AgNOR protein and BrdU-LI scores was found within both the aneuploid and euploid groups.