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Lectins play relevant roles in the innate immunity of invertebrates. From the haemolymph of the prawn Macrobrachium americanum Spence Bate, 1868 we purified by affinity chromatography a lectin (MaL) composed of three subunits (86.6, 66.2 and 42.7 kDa), as identified by SDS-PAGE. It is mainly composed of Gly, Ser and Arg, and Ala, Glx and Lys in a minor proportion; it lacks Trp, Cys and carbohydrates. MaL agglutinated mainly rat erythrocytes, rabbit or mouse erythrocytes were agglutinated with lower capacity; whereas erythrocytes from human or other species were not agglutinated. It is specific for N-acetyl-neuraminic acid (sialic acid), N-acetyl-d-glucosamine and N-acetyl-d-galactosamine; sialylated fetuin and bovine submaxillary mucin are also powerful inhibitors of the lectin’s activity. After physical daily manipulation of prawns for 7 days, the lectin concentration and the specific activity (haemagglutinating activity/protein concentration) increased more than four-fold over the control group (those animals that had not been manipulated until bleeding), suggesting that increased lectin concentration and activity after manipulation could be considered as markers of stress.