The human galectin-3 is a galactoside-binding protein of 31 kDa which functions as a receptor for glycoproteins containing poly N-acetyllactosamine side chains and as a substrate for matrix metalloproteinases-2 and -9. We studied its expression by flow cytoflourimetry, Western, Northern and Southern analyses, in five cultured human breast carcinoma cell lines previously characterized as nontumorigenic, poorly metastatic or metastatic in nude mice. The expression of galectin-3 correlated with the reported tumorigenicity of the cells. The introduction of recombinant galectin-3 into the null expressing non-tumorigenic BT-549 cells resulted in the acquisition of anchorage-independent growth properties in all and tumorigenicity in 3/4 sense transfected cell clones. The data indicate a relationship between galectin-3 expression and malignancy of human breast carcinoma cell lines.