In 42 normal and 65 perinicious anaemia (PA) sera, no significant difference has emerged between the B12 binding capacity. However, the range of B12 binding in PA sera has been found to be much wider, and it is suggested that this may be a source of error in detecting intrinsic factor (IF) antibodies. To eliminate this error, a new modification of the charcoal adsorption method of IF antibody detection is described.