Merthiolate is an organic mercurial widely used as a preservative. Allergic eczematous reactions occur frequently in countries where merthiolate is used as a disinfectant. In Sweden where merthiolate is used mainly for incorporation into vaccines and test agents, the frequency of contact allergy to merthiolate is 3.7% in a 5 yr eczema material. The peak incidence occurs in the age group 20-30 yr. The contact allergy may appear epicutaneously as well as intracutaneously and the histological responses agree with those of an eczematous contact allergy and of a tuberculin reaction, respectively. Cross-sensitization occurs to a few organic mercurials, but not to inorganic or metallic mercury. Merthiolate allergy is frequently correlated to the presence of the clinical picture of pompholyx, i.e., vesicular eruptions of palms and soles. Hypersensitivity to merthiolate occurs even more frequently in healthy young subjects, e.g., in 16% of the military recruits. The intra-pair reaction to merthiolate among monozygotic twins is similar to that of dizygotic twins. An opportunity for sensitization to merthiolate is provided in connection with intracutaneous testing with tuberculin containing merthiolate as a preservative. Tuberculin could act as an adjuvant during sensitization to merthiolate. At present, the iatrogenic occurrence of merthiolate allergy in the Swedish population does not result in eczematous reactions-only in a false-positive cutaneous tests.