Adults of both sexes were exposed to p-xylene (1330207) vapor concentrations of 0, 20, 100, and 150ppm for periods of 1, 3, and 7.5 hours in a controlled environment chamber for two purposes: 1) to develop a practical biologic test which could be used to limit the magnitude of an industrial exposure; and 2) to monitor the physiological response of healthy, sedentary adults to different vapor concentrations and durations of exposure. Repetitive vapor exposure to the current threshold limit value of 100ppm produced no serious subjective or objective health responses in the 16 subjects, nor in eight male subjects who were exposed for 5 days to 150ppm p- xylene vapor. There was indication of the saturation of metabolic pathways of p-xylene when four subjects exercised briefly while breathing 150ppm; however, the certainty and nature of this response requires confirmation. Analysis of methyl-hippuric-acid (495692) metabolite in 24 hr urine samples, and of p-xylene in post exposure blood, saliva, and breath samples all revealed the certainty of exposure to p-xylene vapor. For greatest practicality in routine biologic monitoring, and for establishing limits to p-xylene exposures in workers, it is recommended that breath sampling be carried out. An alveolar sample obtained 15 minutes after p-xylene exposure, should have p-xylene concentrations no greater than 4.5ppm in males or 3.5ppm in females. Concentrations below these limits indicate that workers had not been exposed to deleterious concentrations of p-xylene during the previous 8 hours.