The proliferation associated antigens, Ki-67 and proliferating-cell nuclear antigen (PCNA), have been widely used in studies assessing the growth fraction in human malignancies. It remains unclear, however, whether these markers yield similar assessments of proliferative activity in any given neoplasm. In this study, we compared Ki-67 and PCNA expression in 93 malignant solid neoplasms using bivariate flow cytometric analysis of these antigens and DNA content. The growth fractions measured by Ki-67 and PCNA were compared and correlated with acridine orange (AO) analysis and tumor grade. Our results indicate a significant difference between Ki-67 and PCNA values in neoplasms of low and intermediate grade (P = 0.002); Ki-67 values were significantly lower than those obtained by PCNA in this group. No statistical difference between Ki-67 and PCNA values was found in high grade neoplasms (P = 0.38). Analysis of different cell cycle compartments indicates that the observed difference in the positivity of these markers was due to their differential expression in the G0/1 segment of the cell cycle. We conclude that Ki-67 may better reflect the proliferative activity in solid neoplasms than does PCNA.