The oxime HI-6 dichloride [l-(2 hydroxyiminometh yl-1 -pyridino)-3-(4-carbamoyl-l-pyridino)-2oxapropane dichloride monohydrate] has shown to be a potent reactivator of cholinesterase activity and may have efficacy for the treatment of organophosphate intoxication [SIPRI, 1976;Schenk et al.; Arch Toxicol 36:71-81, 19761. As part of a p r e clinical safety assessment program, the genetic toxicology of HI-6 dichloride was evaluated in a series of assays designed to measure induction of gene mutations and chromosomal aberrations. HI-6 dichloride gave negative responses in the Salmonella mutagenicity assay and in the CHO/HGPRT gene mutation assay. Dosedependent increases in the frequency of chromosomal aberrations were noted when HI-6 dichloride was tested in cultured CHO cells and in cultured human peripheral blood lymphocytes. The mouse lymphoma gene mutation assay, reputed to measure both gene mutations and chromosomal deletions, was negative in the a b sence of metabolic activation. Depending on the criteria employed, a negative or equivocal response was seen in the presence of rat liver-derived S-9 mix. An in vivo rat bone marrow metaphose assay performed to further investigate the in vitro clastogenic responses was negative. The results from these studies indicate that HI-6 dichloride does not induce gene mutations in vitro; however, it is clastogenic in vitro but does not appear to be claste genic in vivo.