Hexahydrophthalic anhydride (HHPA) and methyltetrahydrophthalic anhydride (MTHPA) belong to the group of the acid anhydrides and, among other applications, are used in the production of epoxy resins. These substances are known as potent low-molecular allergens and induce predominantly type I allergies according to Coombs and Gell. We examined 110 employees exposed to HHPA and MTHPA. With all of them a RAST was carried out with the commercially available conjugates of phthalic anhydride (PA) and a skin prick test with 1% and 5% acetonic solutions of PA. In 109 of these sera a radio allergo sorbent test (RAST) was carried out with the not commercially available conjugates of HHPA and of MTHPA. With complaints connected with the workplace the working materials used (HHPA, MTHPA) were also checked by means of the skin prick test. With at least one positive immunological finding (in the RAST and/or skin prick test) in connection with complaints at the workplace, we performed a workplace-related inhalation test under experimental conditions. Specific IgE against acid anhydrides was detected in a total of 17 (15.4%) persons. In the challenge test, six (5.4%) sensitisations were shown to be clinically relevant. On inclusion of borderline positive findings with PA conjugates the RAST produced three false negative and one false positive finding compared with a RAST with HHPA and MTHPA conjugates. With the conjugates of trimellitic anhydride, in no case could specific IgE be detected. The skin prick test led, in comparison with the RAST, to three false positive and three false negative findings.(ABSTRACT TRUNCATED AT 250 WORDS)