In this paper, several studies using different transgenic plants to evaluate the persistence of their products and their effects on soil and plant microorganisms were considered. These studies used cotton, potato, tobacco and alfalfa engineered for the production of pesticidal products (Bacillus thuringiensis endotoxins and proteinase inhibitors) and industrial compounds ( alpha -amylase and lignin peroxidase). Recommendations are made for the design of risk assessment studies on transgenic plants.