In this study we show, by immunoblotting, that Mab B3, a newly isolated monoclonal antibody, reacts with a variety of glycoproteins with different molecular weights expressed in gastric, pancreatic and colorectal cancers. The reactivity pattern differed in cancers arising in different tissues, though no correlation was observed with the histopathological characteristics of the lesions analysed. MAb B3 does not react with liver, brain or kidney cancers and has a limited reactivity with lung cancers but reacts very strongly with metastatic lesions. Because of the limited reactivity of this antibody with normal tissues, MAb B3 genetically fused with toxin in the form of a recombinant immunotoxin may be useful in treating certain kinds of cancer such as metastatic lesions. However, until current trials are completed, we will not know whether this immunotoxin will be helpful in cancer treatment.