Summary: Several nontoxic univalent benzylpenicilloyl (BPO) haptens, a toxic univalent BPO hapten, a divalent BPO hapten and a multivalent BPO hapten were quantitatively compared in regard to their abilities to evoke passive cutaneous anaphylactic (PCA) reactions in guinea pigs sensitized by intravenous injection of rabbit anti-BPO sera. It was found that the four nontoxic univalent haptens were incapable of evoking PCA, that the toxic univalent hapten could evoke only faint PCA reactions, and that the divalent and multivalent haptens evoked equally intense PCA reactions in animals sensitized with antibodies of comparatively high binding affinities, whereas the divalent hapten was a less effective elicitor than the multivalent hapten in animals sensitized with antibodies of comparatively low binding affinities. Evidence was obtained which demonstrates that the divalent hapten did indeed function as a divalent hapten, and not as a toxic univalent or as an aggregated multivalent hapten. These results are interpreted as indicating that, at least in this system, for the effective elicitation of PCA reactions mediated by comparatively high binding affinity antibodies, the elicitor must contain at least two haptenic groups per molecule. For effective elicitation of PCA reactions mediated by antibodies of comparatively low binding affinity, the elicitor must contain more than two (possibly in the range of three to six) haptenic groups per molecule.