Colour developing agents, derivatives of p-phenylenediamine, can cause contact allergy. Patch test reactions to more than one colour developer are sometimes seen in patients. To study whether this is due to simultaneous sensitization or cross-reactivity, guinea pig maximization tests (GPMT) with CD-2, CD-3 and CD-4 were carried out. 5 experiments were performed, using pet. or water as vehicles. When pet. was used, the challenge concentrations could be raised and cross-reactivity between the colour developers, but not with p-phenylenediamine-dihydrochloride, was revealed. When water was used as vehicle, the challenge concentrations were limited because of staining of the test sites and irritation. CD-2, CD-3 and CD-4 were found to be extreme sensitizers according to the classification by Magnusson and Kligman. The importance of using an appropriate vehicle to obtain optimal conditions for the GPMT is stressed. To study the purity and stability of the chemicals used, analysis by HPLC of the test substances at different stages of the GPMT procedure was performed. Aqueous solutions of the colour developers were found to be unstable, while pet. mixtures were stable.