A group of 145 workers exposed to methyltetrahydrophthalic anhydride (MTHPA) was investigated. They were working in a plant which, since 1983, handled an epoxy resin with MTHPA as a hardener. Specific IgE antibodies (RAST) to a conjugate between MTHPA and human serum albumin (HSA) were statistically significantly increased (P= 0·001; 26 subjects = 18% positive) in the exposed group, compared to a non-exposed control group (n= 33). One positive worker was only exposed for 2 months. Twenty-three exposed subjects were also skin-prick test positive to MTHPA-HSA. The exposed group was divided into three different exposure categories, according to their contact with the epoxy resin. The average exposure levels at the time of the investigation were, in zone 10·085 mg/m3, in zone II0·014 mg/m3, and in zone III 0·010 mg/m3, though the exposure probably had been higher earlier. There was an association between exposure intensity and RAST-positive persons (P= 0·0025, chi-square trend test). Forty-four persons (30%) were smokers, and 16(11%) atopics. No association between sensitization and either atopy or smoking was found. There was an association between exposure intensity and specific IgG antibodies (P= 0·0003, chi-square trend test). Specific IgG4 antibodies were closely related to specific total IgG antibodies (P= 0·0001). These findings demonstrate that MTHPA is a sensitizing agent at low levels of exposure.