A novel mammalian lectin activity responsive to monocytic differentiation is described in the human promyelocytic leukemia cell line HL-60. Glycoprotein binding indicates that the lectin recognizes both N-acetylneuraminic acid and galactose-terminating biantennary oligosaccharide structures. Lectin activity is independent of calcium and appears to reside in a Mr 17,000 intracellular membrane protein. Induction of wild-type HL-60 cells into their macrophage-like counterparts by 1,25-dihydroxyvitamin D3 markedly enhances lectin activity. Induction of granulocytic differentiation by retinoic acid does not affect expression of the lectin. HL-60 sublines which are resistant to granulocytic differentiation by retinoic acid, dimethylsulfoxide, or 6-thioguanine are largely deficient in orosomucoid-binding activity. Induction of monocyte/macrophage differentiation of these sublines upregulates lectin activity to the level seen in induced wild-type cells.