A number of fucose containing glycolipids (fucolipids), which are similar in composition to those of human normal and malignant gastrointestinal tissue, have been isolated from whole small intestines of individual dogs. Dogs from which these fucolipids were isolated fell into two types according to the qualitative sugar composition of their fucolipids. Glycolipids from type I dogs contained glucose, galactose, glucosamine, galactosamine and fucose, while those from type II dogs contained the same sugars but lacked galactosamine. Fucolipids isolated from type I and II dogs were tested for both canine blood group and human A, B, H and Le(a) and Le(b) blood group activity. At the concentrations tested, only human blood group A activity was found in significant amounts, and only in those fucolipids which contained galactosamine (type I dogs). Of the fucolipids with human blood group A activity, some had activity comparable to that of glycoprotein blood group substances, while others had lower, but significant, activity. These latter fucolipids also had marked chromatographic differences, indicating that they are of several different structural types, a finding similar to the A active glycolipids of human red cell stroma. None of the isolated intestinal fucolipids had canine blood group activity. A fucolipid with Le(a) activity was also isolated in relatively large amounts from a normal human whole small intestine.