The concanavalin A (Con A) reactivity of malignant and normal human mammary epithelial cells in culture was determined with a hemadsorption assay. Human erythrocytes were treated with various concentrations of Con A, and these indicator Red Blood Cells were incubated with the test cells in situ in culture dishes. The Con A concentration at which approximately 50% of the test cells adsorbed erythrocytes ([Con a] 1/2 max) was determined. Five malignant epithelial cell lines and the primary cultures derived from 3 pleural effusions and 20 solid tumors were tested. Primary cultures of normal epithelial cells were established from human milk samples obtained from 3 separate donors. The average [Con A] 1/2 max value for the 5 cell lines and the pleural effusion cultures was 6 and 5 microgram/ml, respectively. The average [Con A] 1/2 max value for the 20 solid breast tumors was 20 microgram/ml. In contrast to the malignant cells, normal mammary epithelial cells did not adsorb erythrocytes coated with as much as 100 microgram Con A per ml. These results show that Con A reactivity distinguishes normal from malignant human mammary epithelial cells.