In persons occupationally exposed to ethylene oxide, i.e. under the conditions described by Dunkelberg and Hartmetz (1977), the degree of alkylation in histidine of hemoglobin was determined. Quantitative determination of N-3-(2-hydroxyethyl)histidine by mass fragmentography and by ion-exchange amino-acid analysis gave consistent results. Data are in agreement with the fast elimination from tissues (lambda = 4.6 hr-1, i.e. biological half-life about 9 min) found in the mouse. At the respiration rate of light work, and exposure dose of 1 ppm/hr results in a tissue dose that is estimated to involve a risk amounting to 1.101 mrad-equivalents of stochastic effects with a genetic mechanism.