PESTAB. The results of a pesticide test program conducted by the Zentrallaboratorium fur Mutagenitaetsprufung of the Deutsche Forschungsgemeinschaft are reviewed, along with relevant results published by other groups. Although DNA alkylation has not been unequivocally demonstrated in vivo, the organophosphorus insecticides are chemical alkylating agents. Induction of gene mutations in microorganisms in vitro has been unequivocally demonstrated with bidrin (dicrotophos), dichlorvos, dimethoate, methyl parathion, and oxydemetonmethyl. The primary step in the production of microbial mutants is alkylation of DNA bases, and misrepair of the primary lesions appears to be an additional requirement. A different, indirect mechanism, possibly acting via protein alkylation, is probably involved in DNA disintegration by dichlorvos. Although dichlorvos has yielded negative results in mammalian mutation tests, assessment of the potential safety of this compound with regard to genetic hazard should not be made exclusively on the basis of these negative tests. For this assessment, the available data on DNA alkylation and mutagenesis in microbes and the effects on mammalian metabolism should be considered. It is unlikely that dichlorvos, under conditions of normal use as an insecticide, can exert significant mutagenic effects in mammals. Mutation tests with malathion and malathion acid in mammals will be important for a judgement on malathion. The potential mutagenicity of bidrin, bromophos, diazinon, dimethoate, fenitrothion, methyl parathion, oxydemetonmethyl, parathion, trichlorfon, and p-nitrophenol for humans cannot be judged, at present whereas the mono- and di-esters of phosphoric acid probably do not constitute a genetic hazard.