Antibodies cross-reacting with rat gastric parietal cells and the brush border of renal tubules have been demonstrated at a serum dilution of 1 in 5 in 14% of 262 human subjects. The highest incidence was found in patients with autoimmune thyroiditis (27%) or pernicious anaemia (22%); normal group AB blood donors showed an incidence of 18%. Studies with enzymes and solvents suggested that the antigen might be a protein, perhaps masked incompletely by lipid. The antibody also reacted with guinea-pig gastric parietal cells and kidney. The inability to react with human or mouse gastric parietal cells clearly distinguishes the antibody from that found commonly in patients with pernicious anaemia.