(1) Three types of experiments showed that histamine is formed during the reaction of antigen (D. pteronyssinus) with leucocytes of allergic human subjects: (a) total histamine of leucocytes (extracted + released) was increased; (b) histamine-forming capacity (HFC) of leucocytes was stimulated by antigen in vitro; (c) HFC of leucocytes was stimulated by injecting antigen in vivo. (2) Stimulation of HFC of leucocytes by antigen may provide a new and sensitive (though cumbersome) in vitro test of allergy.