The expression of the human cyclin B1 gene was investigated with Western blot analysis in human colorectal carcinomas and in adjacent non-neoplastic colorectal mucosas. Out of 41 cancers, 36 (88% of patients) showed much higher expression of cyclin B1 than did the non-neoplastic mucosa. Proliferating-cell nuclear antigen (PCNA) immunohistochemistry revealed that the labeling indexes of these cancer tissues were 47.3 +/- 11.3% while those of the mucosa were 15.6 +/- 5.5%. Only 5 cancers (12% patients) demonstrated the same expression level of cyclin B1 as the mucosa; however, the PCNA labeling indexes were 42.3 +/- 11% for the cancer tissue, compared to 12.6 +/- 2.4% for the mucosas. Southern blot analysis showed that there was no change of the cyclin B1 gene at the somatic DNA level in spite of its high expression at the protein level. These results proved that majority of colorectal cancers express high levels of cyclin B1, consistent with a high rate of cell proliferation, whereas a small fraction of these cancers lose control of cyclin B1 expression, diverging from their fast cell proliferation.