This paper considers the practical evaluation of the exposure of workers to benzene vapour. It is principally concerned with relating the results obtained from modern sampling techniques to the current criteria for assessing the hazard. The three aspects considered are: sampling the air the man breathes while at work; measuring benzene in the breath he exhales after work; and determining increase in phenol content of his urine. The relationship of these measurements is discussed and the need to develop a mathematical model for describing benzene retention in critical organs of the body, and its subsequent elimination, is indicated.