In vitro challenge tests of leucocytes from patients with intrinsic asthma have shown increased amounts of histamine release on challenge with anti-IgE in 60% and anti-IgG in 30%. These results suggest that the leucocytes of patients with intrinsic asthma are coated with increased amounts of IgE and to a lesser extent IgG in the same proportion and amount as is found in patients with extrinsic asthma. However, in contrast to extrinsic asthma, total serum IgE levels were normal in nineteen out of twenty cases. There were no clinical differences detected between those groups of patients with and without excessive histamine release in the reversed leucocyte anaphylaxis, except that all of the small number of sera in which autoantibodies of various types were detected showed absence of excessive response in this test. It is suggested from this study that reaginic cell sensitization by IgE and less frequently IgG is similar in extrinsic and intrinsic asthma in the majority of patients.