Lectins are proteins or glycoproteins that selective hind sugars. The inhibition of hemaglutination activity has been used as a detection probe for this molecules. Agglutination assays were done with human erythrocytes O, A and B positive groups, testing 44 algal extracts (32 species) as aglutinants. Four extracts showed evident agglutination, 3 Chlorophyta: Codium giraffa Silva, Codium istmocladum Vickers and Ulva lactuca Linnaeus, one Rhodophyta Hypnea spinella (C. Ag.) Kützing. The inhibition of hemagglutinat activity in Codium giraffa, Codium isthmocladum y Ulva latuca were due to N acetylglucosamine. No inhibition was observed by none of the sugars used in this work for Hypnea spinella extract
