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Mercury derivatives are frequent contact allergens and their cross-reactivity is not constant. Thimerosal is an organic mercurial used as an antiseptic and as a preservative in most vaccines.To evaluate cross-reactivity, exposure factors, and tolerance to vaccines containing thimerosal in patients sensitized to mercury derivatives.Design: Observational study (cross-sectional); Patients: 125 patients were recruited for the study, 72 women and girls and 53 boys and men, average age 18.7 years old, range 3 to 65, with positive patch tests to mercury derivatives and/or thimerosal; Interventions: All patients were studied by means of enquiry, patch tests, intradermal tests, and intramuscular challenge with thimerosal.A sensitization to thimerosal was observed in 57 patients. Twenty-four of these 125 patients presented a positive intradermal reaction. Ammoniated mercury seems to be a good marker of mercury sensitization eliciting positive reaction in 78% of all patients and merbromin in 66%. In most cases, (100/125) cross-reactivity was found among mercury derivatives. The intramuscular injection of thimerosal induced a mild local reaction in only 5 patients (4% of the total, 9% of thimerosal positive reactions). Childhood vaccinations, merbromin used as an antiseptic, broken thermometers, and the use of drops were the main sources of exposure.The majority of the patients showed positive tests to both organic and inorganic mercury derivatives. Vaccination with thimerosal is relatively safe, even for individuals with delayed type hypersensitivity to this chemical, since more than 90% of allergic patients tolerated intramuscular challenge tests with thimerosal. A simplified protocol of patch tests to study mercury derivatives is proposed. It would be advisable to restrict the use of mercurial antiseptics and mercury thermometers.