An indirect test for allergy diagnosis is described which uses mast cells from normal albino rats as indicator. These mast cells were brought in contact in vitro with patient''s serum and the suspected antigen (allergen), and the number of degranulated or "fired" cells noted and compared with that in controls. Results with this modified test compare favorably with those obtained by Shelley''s method, which uses rabbit basophil cells. The mast cells keep longer than the basophil cells and the performing of the test is easier.