Thimerosal
Thimerosal is a preservative sometimes used with certain vaccines. Out of the eight manufactured DPT vaccines, only three ever contained thimerosal. Currently, seven out of the eight DPT vaccines on the market do not use thimerosal, and the product that does (Tripedia) contains trace levels, less than 0.3 micrograms per dose. The World Health Organization has concluded that there is no evidence of toxicity from thimerosal in vaccines.
Read more about this topic: DPT Vaccine