Using a combination of two-dimensional gel electrophoresis, silver staining, and a 16-quadrant grid system, we established a set of composite patterns for the colon tumor cytosol proteins of well, moderately, and poorly differentiated tumors. These composite patterns were found to be characteristic of the three individual differentiation classes for colon tumors. The apparent relative molecular masses (Mr) of the resolved proteins ranged from 14,000 to 105,000 and their isoelectric points from pH 5.2 to 8.4. Although the vast majority of the proteins identified in the composite patterns were common, a comparison based upon these patterns revealed two qualitative and seven quantitative protein differences. The qualitative differences were identified by apparent Mr X 10(-3)/pI coordinates of 73/7.2 and 66/6.2. Quantitative differences were identified by Mr X 10(-3)/pI coordinates of 71/6.0, 59/6.7, 57/6.7, 56/5.4, 52/6.1, 30/5.8, and 18/6.2. These cytosolic differentiation marker proteins may facilitate the diagnosis, staging, and monitoring of human colon cancer.