Glycolipids containing fucose (fuco-lipids) have been shown to constitute an individual specific class of compounds both in humans and dogs. Since only trace amounts of some fuco-lipids can be isolated, immunologic techniques are invaluable in identifying these compounds and giving some insight into their structure. By using human and rabbit antisera and ulex lectin with precipitation and agglutination techniques, fuco-lipids isolated from human small intestine were shown to have human A and Lea blood group activity. Fuco-lipids isolated from dog small intestine had human A, H-like and Leb-like blood group activity. With fluorescein-labeled rabbit antisera it was demonstrated that Forssman activity was associated with the lamina propria whereas Leb-like activity was in the glandular epithelium of the small intestine.