The effect of the monocyte locomotion inhibitory factor (MLIF) produced by Entamoeba histolytica is diminished, if not cancelled, when human monocytes are pre-exposed to concanavalin A or sodium periodate, respectively, but not when MLIF is pretreated with sodium periodate. When the MLIF-inhibited monocyte locomotion assays were performed in the presence of 12 different carbohydrates, only the runs containing D-mannose, 4-O-beta-galactosyl-mannoside or mannan revealed a significant reduction in the inhibitory effect. Finally, the MLIF activity was virtually absorbed out with mannan-coupled Sepharose 4B beads. This suggests that D-mannose constitutes an essential part of the receptor for MLIF on the human monocyte membrane.