Summary: Two methods are outlined for studying quantitatively the reagins in the serum of hay fever patients. The reagin content of serum of patients with early hay fever was found to be proportional to the skin sensitivity in 27 of the 29 patients. With the method of serum dilution, the A sera have about four times the sensitizing power of the B sera, which, in turn, have about four times the sensitizing power of the C sera. The ratios are distinctly greater with the method of atopen dilution. In 2 cases with disporportion of reagin content and skin sensitivity, there was a loss of reagins from the serum with no corresponding drop of skin sensitivity. In each case, this decrease of reagins was associated with a gradual decrease in severity of the symptoms. Quantitative study of the reagins in 6 cases of late hay fever indicate that a similar proportion of reagin content and skin sensitivity exists in late hay fever.