The remarkable rise in incidence rates of delayed reactions to 0.1% thimerosal in petrolatum, the sometimes difficult evaluation and the poor clinical relevance of many of these reactions and their preponderance in young-age classes prompted us to investigate the frequency of positive patch tests to 0.05% and 0.01% thimerosal in a group of 155 subjects who had previously reacted positively to 0.1% concentration. The incidence of positive reactions to other mercury compounds was evaluated also in 124 of our subjects, to thiosalicylic acid in 32 and to intradermal testing with 0.0001% thimerosal in saline in 102. The results of the investigation indicate that the optimal eliciting patch test concentration for studing thimerosal sensitivity is 0.05% because it allows for a reduction in irritant reactions. Moreover, in subjects sensitive to 0.05% thimerosal the intradermal testing was more often positive and crossreactions with other mercurials more frequent
