Canine gastric secretions, both fluid acid and viscous non-acid, and hexosamine-rich fractions derived therefrom, were assayed for blood group substance activity in canine and in human test systems. None of the specimens tested was active in the canine tests systems, but all exhibited A and H activity in the human test systems; no human B activity was detected. Hexosamine-rich fractions obtained by peptic digestion and phenol extraction of canine gastric secretion were largely of the neutral mucoid type, and were active in human A and H systems at the microgram level.