Ten sanitary workers exposed to concentrations of ethylene oxide below 1 ppm were studied to determine whether effects could be observed at low exposure levels. A significant increase in the number of sister chromatid exchanges in cultured lymphocytes was found only for five subjects with relatively high exposure in the sterilization area. However, it was not possible to separate clearly the effect of smoking from that of ethylene oxide exposure. No increase in the frequencies of micronuclei in lymphocytes and buccal cells was found. The level of 2-hydroxyethyl adducts to the N-terminal valines in hemoglobin responded in a reliable fashion to chronic ethylene oxide exposure and smoking. Furthermore, measurement of levels of 2-hydroxyethyl adducts to the N-terminal valines in hemoglobin made it possible to reconstruct the dynamics of a leakage of ethylene oxide which involved three workers.