Insecticidal proteins produced by various subspecies of Bacillus thuringiensis and bacterial transforming DNA bind rapidly and tightly on clays, both pure mined clay minerals and soil clays, and on humic acids extd. from soil. This binding reduces the susceptibility of these biomols., which retain their biol. activity when bound, to microbial degrdn. The persistence of bound insecticidal toxins may enhance the control of target pests, constitute a hazard to nontarget organisms, and result in the selection and enrichment of toxin-resistant target insects. The persistence of bound DNA has relevance to horizontal gene transfer in soil. Because of the large differences in the chem. compn. and structure between these proteins and DNA, as well as between clays and humic acids, these studies can serve as models for the potential fate and effects of other biomols. that will be introduced to soil from 'factories' of transgenic plants and animals genetically engineered to produce vaccines, hormones, antibodies, toxins, pharmaceuticals, and other bioactive compds.