Jones-Mote reactivity, defined as a delayed-type skin reaction, occurs transiently early in the course of immunization with protein antigens or hapten conjugates with or without the adjuvant effect of tubercle bacilli. The skin reaction is typically a flat, well-circumscribed erythema with little induration beginning at about 6 hr, reaching a peak at 18-24 hr, and fading or gone at 48 hr. Immunogenic carrier requirements for hapten-specific Jones-Mote hypersitivity resemble those of antibody production rather than of classic delayed hypersensitivity. Skin test antigen requirements indicate that the Jones-Mote reaction involves an active stimulatory response rather than combination with preformed antibody, since ABA conjugates of nonimmunogenic D-polymers do not work. Studies with ALS and carrageenan suggest that the lymphocyte is an important contributor to the reaction, but the macrophage is not. Because the reactions studied here are operationally different from those described by Jones and Mote and because they have a characteristic histology, the term "cutaneous basophil hypersensitivity" is proposed.