To determine whether protein tyrosine kinase (PTK) could be utilized as a biological indicator of colonic tumorigenesis, we examined the PTK activity in the cytosolic and particulate fractions of homogenates from 25 adenomas (polyps) and from adjacent normal mucosa. The particulate fraction PTK activity in the colorectal adenomas was significantly higher than in the normal mucosa. The adenomas were then analyzed according to size, histological type (tubular and tubulovillous), and degree of dysplasia (mild, moderate and severe). The average particulate fraction PTK activity ratio (adenoma/normal mucosa) of tubulovillous adenomas or adenomas with moderate to severe dysplasia was significantly elevated in comparison with that of tubular adenomas or adenomas with mild dysplasia. The particulate PTK activity ratio increased significantly with increasing adenoma size, while the cytosolic ratio did not. The cytosolic PTK activity ratio in tubulovillous adenomas or adenomas with severe dysplasia decreased significantly with increasing size. These findings suggest that colonic carcinogenesis might be associated with alterations in the cellular level of PTK activity and that the PTK activity ratio (adenoma/normal mucosa) in the particulate and/or cytosolic fractions may possibly correlate with the risk of malignant transformation.