In an earlier study, we demonstrated that human exposure to ethylene oxide (EO) during sterilization procedures was associated with increased sister chromatid exchanges (SCE) in cultured peripheral blood. To study further toxic and mutagenic properties of EO in vitro, we devised a membrane dosimetry system which can be used to expose lymphocytes in peripheral blood to specified amounts of EO. The system consists of an exposure chamber, cell culture dishes to which are applied plastic membranes of varying diameters and an infrared analyzer used to monitor ambient EO concentrations in the system. Aliquots of EO-exposed media were subjected to gas chromatography for quantitation. Our preliminary analyses of the cultured cells indicate that the membrane dosimetry system we developed is capable of reproducibly delivering quantities of EO to target cells. Elevated SCEs were observed at as little as 10 micrograms/ml (in media) during a 20-min exposure period. A significant dose-response relationship between SCE and EO dose was recorded up to 35 micrograms/ml, the highest dose tested. Lymphocytes at 35 micrograms/ml showed a fourfold increase in SCE compared to control cultures. We conclude that the membrane dosimetry system provides a reproducible method to investigate EO mutagenesis in human cells in vitro and may prove applicable to a wide range of mutagenic gases.