Aberrant crypt foci (ACF) were induced in the colon of F344 rats by s.c. injection of azoxymethane (AOM) twice in a three day-interval and examined after 4 and 12 weeks. The number and crypt multiplicity of ACF in each section of rat colon increased during this period. Histologically, aberrant crypts consisted of proliferating atypical epithelial cells. Cell proliferation of ACF consisting of 4 aberrant crypts [ACF(4)] and 2 aberrant crypts [ACF(2)], and normal crypts in the colon of rats treated with AOM [normal crypts/AOM(+)] or saline [normal crypts/AOM(-)] was investigated by measurement of the mitotic index, proliferating cell nuclear antigen-labeling index (PCNA-LI), and 5-bromo-2'-deoxyuridine-labeling index (BrdU-LI). All three parameters of the cell proliferative activity of ACF(4) were higher than those of normal crypts/AOM(+) and normal crypts/AOM(-). The PCNA-LI and BrdU-LI in ACF(2) were the same as those in ACF(4). These findings suggest that ACF have increased cell proliferative activity. The correlation of these three parameters confirmed that the PCNA-LI is also a useful parameter for evaluating cell proliferative activity in ACF. The presence of many cells stained by PCNA in the upper portion of ACF suggested that ACF have more G1 phase cells, which readily respond to mitogenic stimulation, than G0 phase cells, which are predominant in normal crypts.