Non-Hodgkin's lymphomas are characterized by heterogeneity in cell kinetics, which may be related to differential expression of proliferation-associated nuclear antigens. We have performed two-color flow cytometric quantitation of nuclear antigen p105 and DNA content in nuclear suspensions from 80 paraffin-embedded lymphomas. High-grade lymphomas with diploid DNA histograms tended to express high levels of p105 in all phases of the cell cycle. Aneuploid populations, observed in 14 of 80 cases, showed high p105 expression compared with diploid cells of the same case, with the highest aneuploid:diploid p105 ratios in the high-grade group. We conclude that high-grade or DNA aneuploid lymphomas tend to have increased expression of p105; but considerable variations within each histologic subtype were observed, and the differences between histologic grades were not statistically significant. Further studies that correlate p105 expression and clinical outcome will help to determine the prognostic value of this marker.