Hemoglobins of the Diptera species Chironomus thummi thummi (Chi t I) are potent inhalant allergens. Chi t I-specific histamine release was measured by a new radioimmunoassay in whole human blood taken from 20 sensitized patients, 11 exposed nonsensitized probands, and 11 nonexposed controls. The sensitized patients, who all had positive skin tests and radioallergosorbent test results with Chi t I, showed a significantly higher histamine release than the two other groups. However, within the patient group, the percentage of released histamine did not correlate with the intensity of the skin test response or the concentration of Chi t I-specific IgE antibodies. Our results demonstrate that this method is a sensitive and specific in vitro test for evaluation of IgE-mediated sensitization.