THE FREI intradermal reaction used for the diagnosis of lymphogranuloma venereum is, like the tuberculin reaction, a manifestation of delayed-type hypersensitivity. The former reaction is regarded as positive when the test site presents at the end of 48 or 72 hours a papule or indurated area which measures not less than 6 mm. in diameter. Erythematous zones may or may not surround the papule or indurated area, and the size of such zones is a much less reliable measure of the intensity of the reaction than the size of the papule or indurated area. The microscopic architecture of the Frei reaction is not diagnostic of lymphogranuloma venereum. Complement-fixing antibodies appear in the blood serum of lymphogranulomatous persons and can be detected and measured by the use of an antigen identical with that employed in the intradermal test, namely, an inactivated suspension of the virus agent