As previously stated (1) there is a marked variation in individual sensitivity to flea bites and many persons present true allergic manifestations after contact with these insects. Although urticaria is the most frequent symptom, patients hypersensitive to flea bites may also suffer from asthma. Individuals residing in flea-infested areas and subject to repeated exposure to the insect, become densensitized. Therefore in order to simulate natural desensitization, we have prepared a polyvalent flea antigen for use in the hypersensitive individual (2). In the course of our investigations, we studied 128 patients (children and adults); 115 of whom were relieved of their symptoms following administration of the antigen; 10 noted no effect and 3 failed to complete the course of treatment. Three children, all in their third year of life had urticaria due to flea bites, and were relieved after receiving 0.6 cc. of the antigen.