Positive patch test reactions to thimerosal 0.1% pet (40/ 690 subjects: 5.8%) were more common in younger age groups, in the allergic contact dermatitis group and in subjects who had used contact lens solutions. In the 40 thimerosal‐positive patients, the minimum eliciting quantity of preservative was evaluated using different test concentrations: 0.05% and 0.01% pet (patch testing) and 1:10,000 in saline (intradermal testing). Cross‐reactions between thimerosal and other mercury Compounds and sensitivity to thiosalicylic acid were also examined. The results of the investigation demonstrate that many of the reaction to 0.1% thimerosal are probably irritant, because only half the subjects studied had positive patch test when allergen concentrations 5 to 10 × lower than that conventionally used for patch testing, were utilized. In these subjects, the average strength of patch test reactions was higher, intradermal testing was more often positive and cross‐reactions between mercurials more frequent. These data indicate that the optimal eliciting patch test concentration for studing thimerosal sensitivity is 0.0