Pokeweed mitogen (PWM) lectin, known to bind branched poly-N-acetyllactosamines, has a highly selective affinity for human colorectal carcinomas. We performed light microscopic (LM) histochemistry with PWM lectin on paraffin sections of human colorectal tissues. In histological sections, normal mucosae and adenomas with mild dysplasia exhibited negative reaction (0/10, 0/13, respectively) with or without neuraminidase pre-digestion, whereas adenomas with moderate dysplasia showed a small increase in PWM lectin reactivity after neuraminidase digestion (4/23). In contrast, we observed a high incidence of positive reactivity in colorectal carcinoma without neuraminidase pre-digestion (38/44). After digestion with neuraminidase, there was increased reactivity of colorectal carcinomas in situ (7/12) and invasive carcinomas (13/32). These results imply that human colorectal carcinomas consistently contain substantial amounts of PWM-reactive branched poly-N-acetyllactosamine glycoconjugates structures. We also compared the staining patterns of PWM lectin and monoclonal antibodies (MAb) directed to Lewis X (LeX) or Lewis Y (LeY) antigen. PWM lectin reactivity was largely confined to the apical membranes of carcinoma tissues. MAb-LeX or MAb-LeY immunoreactivity was seen on the apical membranes and in the cytoplasm of both adenomas and carcinomas. Therefore, histochemical studies with this lectin should be useful for identification of carcinoma tissues and analysis of glycoconjugates associated with colorectal carcinoma.