Summary Thirteen typical cases of ragweed hay-fever were studied with intracutaneous, conjunctival, and serological tests before and after the first three months of treatment with injections of pollen-extract. In 12 of the 13 patients the reaction to intracutaneous tests with ragweed pollen-extract, which had been preserved by the cryochem-process, was less after treatment. In 11 of 13 patients the reaction to conjunctival tests with ragweed pollen-extract was less after treatment. The sera of 12 of the 13 patients showed more skin-sensitizing antibody, as measured by the dilution-test, after treatment. All of the sera required more ragweed pollen-extract to neutralize the skin-sensitizing antibody after treatment. All post-treatment sera inactivated one of the antigenic fractions (fraction 1) of low-ragweed pollen-extract. In only one case did the serum inactivate the second antigenic fraction (fraction 2) of the pollen-extract. None of the sera taken before treatment inactivated ragweed pollen-antigen. Patients who were sensitive to both antigenic fractions of ragweed pollen but who developed antibody inactivating only one fraction did not develop tolerance for large doses of ragweed pollen-extract and had more constitutional reactions than those who were sensitive to only one fraction and developed antibody inactivating that fraction. Sera that inactivated only the more active fraction (fraction 1) of the ragweed pollen-extract showed blocking activity when tested by the neutralization-method. When such sera were tested by the dilution-method, the results obtained depended on the way in which the serum used to sensitize cutaneous sites reacted to the two antigenic fractions of the ragweed pollen.