The effects of chronic inhalation exposure to ethylene-oxide (75218) and propylene-oxide (75569) on sister chromatid exchange and chromosome aberrations were investigated in monkeys. Male cynomolgus-monkeys were exposed to 0, 50, or 100 parts per million (ppm) ethylene-oxide or 0, 100, or 300ppm propylene-oxide, 7 hours per day, 5 days per week, for 2 years. Blood was drawn during the last month of exposure for sister chromatid exchange and chromosome assays in peripheral lymphocytes. Controls had a mean of 5.4 sister chromatid exchanges per 50 metaphases. Monkeys exposed to propylene- oxide at both concentrations averaged 5.8 sister chromatid exchanges. Ethylene-oxide at 50ppm induced 10.2 sister chromatid exchanges, whereas the rate for those exposed to 100ppm was 15.1. In controls an average of 0.6 percent abnormal chromatid and chromosome type aberrations were seen, a rate similar to that for animals exposed to propylene-oxide at both concentrations. Animals exposed to 50ppm ethylene-oxide had an average of 2.0 abnormal cells and those exposed to 100ppm had an average of 3.7 abnormal cells. Control cells had breaks, acentric fragments, and dicentric cells. In addition, peripheral lymphocytes from animals exposed to ethylene- oxide showed triradials, quadriradials, and complex chromosomal rearrangements. The authors conclude that chronic inhalation exposure of monkeys to ethylene-oxide produces dose dependent increases in the incidence of sister chromatid exchanges, and chromatid and chromosome type aberrations in peripheral lymphocytes. Propylene-oxide does not produce such genetic alterations. The permissible exposure concentration of 50ppm of ethylene-oxide may not be adequate to prevent adverse human health effects.