Two-dimensional gel electrophoresis has been used to analyze polypeptide composition of specimens of human normal colon mucosa and colorectal carcinomas. Immobilized pH gradient in the first dimension improved the separation and enabled us to analyze proteins from whole tissue samples with high reproducibility. Polypeptides were separated within sigmoidal 3.5-10 pH gradient and range of mol. weight 15-200 KD. In global, protein patterns of normal and malignant tissue seemed to be close to each other. We attempted to combine two-dimensional electrophoretic separation with the immunoblotting technique using patient sera to identify possible tumor antigens. From a wealth of spots on silver stained protein maps of adjacent normal colon mucosa or tumor tissue, only few spots gave positive reaction. These spots might be classified into two groups: 1) common for normal colon mucosa and tumor, 2) specific for tumor tissue. The extent of patient's antibody reaction with antigens appeared to correspond to urinary neopterin level, an index of immune activation.