An ELISA was applied to measure IgG sub-class antibodies to cow's milk beta-lactoglobulin (BLG), alpha-lactalbumin (ALA) and alpha-casein (AC) and to hen's egg ovalbumin (OA) in the sera of nineteen adult patients with milk intolerance causing either asthma, eczema or both. Results were compared with those of forty blood donors and twenty adult patients with either asthma or eczema due to inhalant allergy. Apart from one blood donor, high titres of IgG sub-class antibodies to all three milk proteins were found only in the milk intolerance group. The most frequently detected antibody was AC-specific IgG4; being high (i.e. greater than 9.98 micrograms/ml) in eight milk intolerance cases: six with eczema, one with asthma and one with both. A variable proportion of these eight patients also had high levels of IgG1, IgG2 and IgG3 antibodies to AC and IgG1, IgG2, IgG3 and IgG4 antibodies to BLG and ALA. In contrast, IgG antibody to the egg protein, OA, was remarkably restricted to IgG4 and was present in high titres in 68.4% of milk intolerant patients, 60% of inhalant allergy patients and 30% of blood donors. However, the greater incidence of high titres of IgG4 antibody to OA, compared to AC, was due to the superior coating efficiency of OA resulting in a more sensitive assay. We conclude that some adult cases of milk intolerance, particularly those with eczema, can be diagnosed by detecting raised serum levels of IgG sub-class antibodies to milk proteins.