Occupational exposure to ethylene oxide (ETO) was studied in ten workers employed in a hospital sterilizer unit by testing environmental air, alveolar air and blood during and at the end of the workshift. Alveolar (Ca) and blood (Cb) ETO concentrations were correlated with each other (r = 0.744, n = 36, P less than 0.001) and both with the environmental (Ci) concentrations (r = 0.947, n = 144, P less than 0.001; r = 0.827, n = 36, P less than 0.001). The alveolar retention of ETO (1-Ca/Ci) was equal to 75-80% of the inhaled ETO. In comparison with a blood/air partition coefficient equal in vitro to 90 (SD = 20), the mean Cb/Ca ratio found in the exposed workers was equal to 12-17. During work the blood ETO concentration was, on average, three times the environmental ETO concentration.