We investigated the ability of tobacco (TX1) cells in culture to activate aflatoxin B1 (AFB1) and benzo(a)pyrene (BAP) using the plant cell/microbe coincubation assay. Initial experiments with Salmonella typhimurium as a genetic endpoint using previously published assay conditions failed to demonstrate that either of these agents could be activated into a mutagenic form by TX1 cells. However, when modifications of the assay, including increased plant cell densities and/or increased preincubation times, were employed, both AFB1 and BAP were demonstrated to be potent plant promutagens. These data demonstrate that, for the purpose of routine screening and toxicity assessment studies focusing upon plant activation, careful attention must be paid to both the chemical or complex mixture under study as well as to the metabolic potential of the particular plant cell system employed. This will ensure a more complete understanding of the problem at hand, provide insight into the mechanisms of activation, and guard against possible false negative results.