In patients hypersensitive to p-tert-butylphenol-formaldehyde resin (PTBP-F-R), and butylphenol derivatives therein, it is for diagnostic, therapeutic and preventive reasons necessary to know the identity of the primary sensitizing substances, their sensitizing capacities as well as their cross-reaction patterns. The monomers in PTBP-F-R, 2-methylol p-tert-butylphenol (2-MPTBP) and 2,6-dimethylol p-tert-butylphenol (2,6-MPTBP), have been shown to be contact sensitizers in man. The aim of this study was to investigate the sensitizing capacities of the monomers and establish cross-reacting patterns in the guinea pig with the guinea pig maximization test. 2,6-MPTBP was shown to be a strong sensitizer while it was indicated that 2-MPTBP was a sensitizer. Animals sensitized to 2,6-MPTBP showed cross-reactions to 2-MPTBP and p-tert-butylcatechol. No cross-reactions were shown to p-tert-butylphenol, tert-butyl 4-hydroxyanisole (BHA) and 3,5-di-tert-butyl 4-hydroxytoluene (BHT).