The "delayed hypersensitive" reactivity induced by antigen-antibody complexes has been studied from the standpoints of the role of such complexes in establishing this state, and the relationship of this state to classical delayed hypersensitivity. It has been shown that the reactivity established by antigen-antibody complexes appears early after injection, disappears within a few days, and is characterized by several properties which make it appear similar to true delayed hypersensitivity, including its appearance, its relative persistence for 48 hours, and its occurrence in the absence of antibodies. By the same tokens, it may be distinguished from hypersensitive reactions of the immediate type. It is referred to here as reactivity of the Jones-Mote type. Antigen alone stimulates exactly the same kind of early reactive state, but with larger doses of antigen this is later replaced by other immunologic responses including circulating antibodies and Arthus reactivity. If sufficiently small doses of antigen are employed, however, the "monophasic" reaction which follows antigen-antibody complexes consisting of the Jones-Mote type of skin responsiveness may be seen. The dermal reactivity under discussion is unlike classical delayed hypersensitivity chiefly in its evanescent character; it is present only during a few days early after antigen administration. It is suggested that this kind of reactivity, which may perhaps require a category of its own, may be related to the "tissue immunity" to tumor transplants which has been observed in mice.