The incidence of mercury sensitivity in millitary personnel without contact dermatitis was investigated using the patch test with 1% ammoniated mercury and 0.1% thimerosal. A questionaire study relating to mercury exposure was also performed in 124 subjects. We also investigated whether the several vaccines curently in use in Korea contain mercury products. The results were as follows . 1. Ammoniated mercury sensitivity was in 10.8% and thimerosal sensitivity was 6.1% in all tested subjects, a similar percentage observed in patch test clinic of the authors. 2. Vaccination history was statistically related significantly, to the senstivity to mercury allergens (P<0.05). 3. All subjects sensitized to thimerosal were also allergic to ammoniated mercury, suggesting crosssensitivity. 4. Hepavax-B, D. P.T., Japanese encephalitis vaccine, and fluvax, currently in use in Korea, contain thimerosal as a preservative. (Kor J Dermatol 1992; 30(3): 347-353)