The Tradescantia genetic system developed by the late Dr. Arnold H. Sparrow for the study of effects of ionizing radiation is applicable to chemical mutagen detection. Early radiobiological data demonstrated that the stamen hairs were sensitive to as little as 0.25 rad of x-rays and that the number of cells showing a phenotypic change in pigmentation from blue to pink plateaus after approximately 21 days of chronic, low-level irradiation. Exposures to the air pollutants SO2, NO2, and O3 and to vapors of mutagens such as 1,2-dibromoethane (DBE) and ethyl methanesulfonate (EMS) demonstrated the usefulness of the system as a detector of chemical mutagens. A significant number of phenotypic changes was observed following exposures to as little as 0.14 ppm of DBE. The maximum sensitivity of the system is obtained with long-term or chronic exposures because the response increases linearly in proportion to the duration of exposure up to 21 days.