(1) The covalent binding of benzo (a) pyrene metabolite, trans 7,8-diol and 9,10-epoxide B(a)P (DE1) to DNA in vitro was significantly increased in the presence of Pb ions, Hg ions, Zn ion and Na ions but not with Ca ions and Mg ions. (2) DE1 binding to the phosphate groups of DNA increased (54%) in the presence of La ions while the majority of the binding occured with guanine. (3) The mutagenesis test with Salmonella typhimurium showed about a 2 to 3 fold increase in His plus mutants with Salmonella typhimurium showed about a 2 to 3 fold increase in His plus mutants when La ions, Mn ions, Pb ion and Ni ions were added to DE1. These metal ions tested have little, if any, mutagenic potential by themselves. Some metals such as Hg ion and Zn ion induced a low molecular weight protein (metallothionein) in African green monkey cell lines. The induction is transcriptionally regulated and Ca ion dependent. The role of the inducible protein in cellular defense against toxicity and muta-carcinogenesis is under investigation.