Chlorocresol is a biocide with widespread use in industry and pharmaceutical products. It is an occasional human contact sensitizer. The sensitizing potential of chlorocresol was judged strong using the guinea pig maximization test (GPMT) and doubtful in the less sensitive open epicutaneous test (OET). When different induction concentrations were used, the results indicated an optimal sensitizing concentration above which no further increase in the sensitization rate occurred. Rechallenge 2 weeks later showed a marked decrease in sensitivity. Consecutive human patch tests with chlorocresol 2% in pet. showed 11 reactions among 1462 patients tested, but none were explainable and reproducible during re-tests and provocative use tests, indicating that the GPMT overestimated the sensitization potential. The results from guinea pig allergy tests cannot stand alone but have to be validated by other sources of information.