Study was made of immune responses in cow's milk allergy by a new immunoassay that measures total Ig-secreting cells and specific antibody-secreting cells during their maturation cycle in peripheral blood. These primed gut-associated lymphoid tissue-derived lymphocytes are assumed to reflect the intestinal immune responses. During diagnostic milk provocation, 15 patients had acute urticarial skin eruptions, eight patients had slow onset of eczema, and 15 showed symptoms from the gastrointestinal tract. A significant increase in IgM-secreting cells (means with 95% confidence intervals) from 382.2 (265, 552) to 621.4 (381, 1013)/106cells,p< 0.01, but not IgA- and IgG-secreting cells was associated with acute urticaria. In patients with eczematous skin eruptions and gastrointestinal symptoms, the response involved all these Ig isotypes. The magnitude of the postchallenge Ig-secreting cell responses in patients with gastrointestinal symptoms in the IgM class [from 657.9 (428, 1012) to 3544.0 (1696, 7406)/106cells,p<0.001] and the IgA class [from 974.6 (590, 1610) to 2482.4 (1528, 4028)/106cells,p=0.001] significantly exceeded that of the patients with cutaneous symptoms. Notwithstanding the distinct increase in the total number of Ig-secreting cells, the specific antibody- secreting cell response specifically directed against β-lactoglobulin and α-casein was small and inconsistent. These findings indicate that immune exclusion of milk antigens is defective in cow's milk allergy. The quality and extent of the response varied in the three reaction types, suggesting that different immunopathogenic mechanisms are operative in cow's milk allergy.