The role of carbohydrate in the interaction of human migration inhibitory factor (MIF) with human peripheral blood monocytes was investigated by studying the effects of different exoglycosidases on the cellular response to MIF. When monocytes were pretreated with neuraminidase, an exoglycosidase specific for sialic acid, they became unresponsive to MIF. Other glycosidases, such as β-galactosidase and α-mannosidase, were inactive in this respect. The effect of neuraminidase was reversible since the response to MIF was restored to normal levels after 24 hr. In parallel studies, a glycolipid-enriched extract from U937 cells, a human macrophage-like cell line, known to enhance the monocyte response to MIF, lost this activity when treated with neuraminidase and α-l-fucosidase, but not with β-galactosidase. This suggests the importance of terminal sialic acid and fucose residues for the interaction between monocyte membrane glycolipids and MIF.