Ras gene protein products (p21) reacting with the monoclonal antibodies ras 11, DWP, R256 and E184 were studied with an immunohistochemical method which was applied to 17 normal and 79 colorectal adenocarcinoma specimens. Normal colorectal epithelium showed positive staining for ras 11 in 35% of the cases, but not for DWP, R256 and E184. The antibodies showed positive staining in colorectal adenocarcinomas in 76, 53, 29 and 35% of the cases respectively. The degree of staining for ras 11 was significantly related to the grade of differentiation and increased from Dukes stage A to C. Strong staining for ras 11 predicted a significantly shorter recurrence-free interval (p less than 0.001). In Cox's regression analysis, the degree of staining for ras 11 was a prognostic factor independent of the grade of differentiation and Dukes stage (p less than 0.01). The results indicate that the enhanced expression of pan ras p21 may provide an important biological marker for determining prognosis in colorectal adenocarcinomas.