Plant activation is the process by which promutagenic agents are activated into mutagens by plant systems. Many promutagens are activated by plants as well as by the familiar mammalian microsomal monooxygenase systems. However, several environmentally important agents are preferentially activated by plant cells. Plants have become a reservoir for the deposition and accumulation of environmental xenobiotics. With the widespread use of agricultural chemicals on crop plants and with the global exposure of plants to pollutants, the possibility that plant-activated agents may be introduced into the human food chain is a cause of concern. Environmentally relevant agents should be evaluated with plant assays. The plant cell/microbe coincubation assay uses cultured plant cell suspensions as the activating system and bacteria or yeast cells as the genetic indicator organism. After a treatment time, the microbes are plated on selective medium. In this way the activation system and the genetic system can be independently studied. In addition, the viability of the plant cells and the microbial cells can be independently determined so that the toxicity of a test agent can be evaluated. We have employed cultured tobacco, cotton, carrot, maize, and Tradescantia cells to study the activation of test agents and complex environmental mixtures. In addition to screening, this assay is being used in basic research to elucidate the biochemical mechanisms of plant activation. The results of experiments using the peroxidase inhibitors acetaminophen and diethyldithiocarbamate showing repression of TX1-cell activation of m-phenylenediamine and 2-aminofluorene indicate that a TX1-cell peroxidase pathway is involved in the plant activation of aromatic amines.