The sensitizing capacities of 2-methylol phenol (2-MP), 4-methylol phenol (4-MP) and 2,4,6-trimethylol phenol (2,4,6-MP) were investigated with the guinea pig maximization test. These compounds are known sensitizers in phenol-formaldehyde resins. The study was performed in order to assess and compare the degree of the sensitizing capacities of these chemically related substances. The animals were also rechallenged with the sensitizer and 5 chemically related compounds in order to study the cross-reaction patterns. 2-MP was demonstrated to be a strong sensitizer, 4-MP a moderate sensitizer and 2,4,6-MP a weak sensitizer. With 2-MP as the sensitizer, o-cresol and 2,4-dimethyl phenol were possible cross-reacting substances. p-cresol and 2,4,-dimethylol phenol were possible cross-reacting compounds for 4-MP. The chemical investigation by high pressure liquid chromatography indicated that the compounds tested were pure and separable.