Manifestations of cellular and humoral hypersensitivity to cromolyn sodium were sought in six asthmatic patients suspected of having acute (three cases) or subacute (three cases) adverse reactions to cromolyn inhalation. Lymphocytes from all patients produced migration inhibition factor (MIF), and those from four patients incorporated increased amounts of 3H-thymidine in response to cromolyn in vitro. Lymphocytes from four of nine cromolyn-tolerant asthmatics demonstrated increased 3H-thymidine incorporation but none showed MIF production, whereas lymphocytes from normal subjects failed to react to cromolyn in either assay. Two of three patients with subacute reactions had increased serum binding of 3H-cromoglycate which was attributable to the IgG fraction. Thus although patients tolerating cromolyn therapy may demonstrate lymphocyte transformation in vitro, only those with clinically apparent adverse reactions produce lymphocyte MIF or possess serum-binding activity for the drug.