In the present study a methodological approach is taken which quantitates aberrant dysplastic crypts in the unsectioned murine colon. C57BL/6J or CF1 female mice (7-8 weeks old) were injected (i.p.) with azoxymethane (5 mg/kg body wt./week) for 4 weeks. Their colons were excised, cut open on the median axis and fixed flat in buffered formalin. Unsectioned colons were stained with methylene blue. The mucosal side was examined under a light microscope. The aberrant crypts, which are larger and have a thicker epithelial lining, were easily visualized using X 4 or X 10 objectives. CF1 mice, which are more sensitive to developing colon tumors, had a higher number of aberrant crypts/colon than their less sensitive counterparts, C57BL/6J mice (5.0 +/- 0.7 vs. 2.4 +/- 0.7). The usefulness of this observation as a possible measure of neoplastic events is discussed in the animal and human situation.