A HPLC method was developed for the determination of histamine released from washed blood cells of allergic patients after exposure to allergens in the presence or absence of plasma. The blood cells used were preserved in a vital state at 4 degrees C during at least 6 hours after blood-collecting. The responses of blood cells to an appropriately diluted series of a mite feces extract (Dff) solution were evaluated in three patient's groups (68 cases of hyposensitized, 59 cases of newly visited and 44 cases of the others) with bronchial asthma. Washed blood cells from the hyposensitized groups had a tendency to release histamine easily even with lower concentration of the Dff solution. Both the sensitivities of washed blood cells to Dff or mite body extract (Dfb) and the responses of the cells to diverse antigens in neither extract differed largely in individual cases. The titer of blocking antibody could be expressed as the amount proportional to the concentration of Dff or Dfb.