Proliferating cell nuclear antigen (PCNA) is a key cycle regulatory protein of known structure and function that also has an important role in DNA repair, its use as a marker of proliferation can be assessed directly using a thymidine analogue in suitably labelled pathological material. Following optimisation studies, a quantitative and cell cycle phase-specific analysis of PCNA expression in fresh biopsies from 50 colorectal tumours (Series A) was undertaken using dual parameter flow cytometry (FCM). For comparison, quantitative histometric studies of PCNA expression were undertaken on 50 archival bromodeoxyuridine (BrdUrd) labelled colorectal tumours (Series B). In FCM assays, PCNA-specific fluorescence was displayed throughout the cell cycle in both cells and nuclei under all preparation conditions, but to a very variable extent. The mean PCNA labelling index ranged from 38.7% to 53.0% according to the method of cell extraction used. In the 27 diploid tumours in Series A, the median PCNA LI in G0/G1 was 71.5% (range 27.0-90.6%), in S it was 10.5% (3.3-29.5%), and in G2/M it was 17.4% (5.7-43.5%). In the histometric studies of Series B tumours, the mean [S.D.] PCNA labelling index (LI) was 38.8 [9.8]%, compared with the BrdUrd (histometric) LI of 21.1 [9.0]%. The BrdUrd LI measured by FCM was 12.4 [6.5]%. PCNA-PC10 is expressed throughout the cell cycle in human colorectal tumour biopsies, which is in keeping with the range of DNA repair, synthesis, and regulatory functions that it is now recognised to perform throughout the cell cycle.