To study the length of exposure time required to elicit para-phenylenediamine (PPD) allergic reactions, patients known to be allergic to PPD were recruited and patch tested. A group of 7 patients were patch tested with 1% PPD in pet. for 15 min, 30 min and for 120 min. The remaining 9 patients were patch tested with 1%, 0.3%, 0.1% and 0.01% PPD for 15 min, 30 min and for 120 min each. With exposure for 120 min, 11 of 16 subjects reacted to 1% PPD and 2 of 9 reacted to 0.01%. With exposure of 15 min, 6 of 16 reacted to 1% PPD and 0 of 9 reacted to 0.01% PPD. This study showed marked inter-individual variability in eliciting a reaction to the PPD molecule on patch testing, with regard to both the exposure time and the concentration required.