Lectins constitute a group of sugar-binding proteins, neither antibodies nor enzymes. This proteins have a significant role in the immune responses of host. They bind specifically to carbohydrate molecules expressed on pathogens and help in their rapid clearance by enhancing opsonization and phagocytosis. The discovery of fish lectins has added a new dimension in lectin biology and fish immunology. Although very little is known about their biological significance, fish lectins appear to play important roles in fertilization and defense against microorganisms. Tambaqui fish, Colossoma macropomum (Cuvier, 1818) is one of the aquatic species more used for human feeding in the North Region of Brazil being largely distributed in its rivers. The aim of this work was the partial purification and characterization of C. macropomum serum lectin (ComaSeL). Fish serum  was diluted (1:1) in 0.01 M Tris-HCl buffer, pH 7.5, and submitted to a fractionation with ammonium sulphate (0-20% and 20-40%). After dialysis, hemagglutinating activity (HA) was evaluated with glutaraldehyde treated rabbit erythrocytes. Also, protein measurement, HA inhibition (HAI, 200, 100, 50 e 25 mM), ion assays (Mn⁺², Mg⁺², K⁺²), and thermal stability at different temperatures (30 a 100 ⁰C) were performed. Electrophoresis for acidic and native as well as denatured and reduced proteins (SDS-PAGE) were made. Fraction 20-40%, highest specific HA, was submitted to ion exchange chromatography (DEAE-Cellulose) and adsorved material was eluted with a saline gradient (0 a 0.5 M, in 0.01M Tris-HCl buffer, pH 7.5). ComaSeL was almost totally inhibited (2^-1) by galactose and fucose. ComaSel activity was not altered with ions; lectin lost activity at 40 °C. DEAE-Cellulose partially purified ComaSel, an acidic lectin which recognizes galactose and fucose carbohydrates and therefore can be included within the fucolectin family.