Abstract: A biologically active oligosaccharide compound comprising at least two L hexose rings connected together by an ether oxygen atom. The ether oxygen atom is connected to a first of the rings at the first carbon atom to the right of the hexose ring oxygen atom. The compound contains at least one sulfate or phosphate group connected to the first ring at the third carbon atom to the right of the ring oxygen or to a methyl group at the fifth carbon atom to the right of the ring oxygen atom.The invention further includes the method for using the above oligosaccharide compound to detect .alpha. 1,3-L-fucosyltransferases or to block the activity of such .alpha. 1,3-L-fucosyltransferases or structures which mimic the structure of such fucosyltransferases to the extent that such structures bind to compounds of the present invention, e.g., as in the case of HIV virus.