Micronucleus formation and chromosomal aberration (CA) in V79 cells were compared for their sensitivity in response to ethylene oxide (EtO) treatment. The results indicate that EtO exposure for 30 min induced CAs in V79 cells at the concentration of 3,500 ppm or higher. A statistically significant difference (P less than 0.01) was found between treated and control groups at all concentrations tested based on percent aberrant cells. The increase was dose-dependent with a correlation coefficient of 0.88. The aberrations found include chromatid and isochromatid breaks, fragments, minutes, and exchanges. Results of the micronucleus assay both in mononucleated and binucleated cells showed a slight but not statistically significant increase in micronuclei with doses between 457 to 4115 ppm. At the highest concentration tested (12344 ppm) EtO caused a significant increase in the micronucleus frequency (P less than 0.05).