An index was devised to estimate the hypersenstivity states of [human] mast cells and basophil leukocytes on the basis of serum concentrations of total IgE immunoglobulins and specific IgE antibodies. Correlation between improvement of blindly determined clinical states and the indices (stability index: SI) was subject to statistical analysis. A close correlation was seen between hyposensitization therapy and the stability index in atopic asthmatic patients.