Wheal-and-erythema responses were studied in normal human volunteers and in a single human subject who is sensitive to the 2,4-dinitrophenyl group. In the normal subjects, reactive skin sites were established by intradermal injection of purified rabbit antibody specific for the 2,4-dinitrophenyl group. In both the active and passively sensitized subjects, wheal-and-erythema was elicited by intradermal injection of a 2,4-dinitrophenyl protein, but not by injection of the same conjugate mixed with certain low molecular weight 2,4-dinitrophenyl haptens or with univalent fragments split by papain from anti-2,4-dinitrophenyl antibody. The latter fragments, unlike intact, bivalent, antibody, do not sensitize normal human skin sites. From these and other observations it is concluded that the wheal-and-erythema response in human skin requires mutually multivalent antigen and antibody. This requirement suggests that multimolecular complexes, containing at least 2 antigen and 2 antibody molecules, are essential in the pathogenesis of this allergic response.