p-Phenylenediamine (PPD) is a commonly used hair-dye and a potent skin allergen. The mechanism of sensitization is unknown, as PPD is protein unreactive. We studied Bandrowski's base (BB), a PPD trimer, as well as 1,4-benzoquinone (BQ), a PPD hapten. PPD patch-test positive patients were patch-tested to BB and BQ. All tests were negative to 0.01% BQ and 0.01% BB. Five of 14 (35.7%) tested had true positive reactions to 0.1% BQ. One percent BQ was found to be irritant. Seven of 43 tested (16%) were positive to either 0.1% or 1% BB. The positive reactions to BB were weak, even when PPD reactions were strong. Mice lymph node assay gave EC3 values of 0.14% for PPD compared with 0.03% for BB. Therefore, BB is approximately 10 times more potent than PPD, taking into account the molarity. We suggest that while PPD may act as a prohapten, there is probably a spectrum of antigenic determinants in vivo. BB may be bound or metabolized by keratinocytes before it reacts with Langerhans cells.