Abstraet--The effect of degassing on the cytotoxicity of an ethylene oxide (EtO)-sterilized copolymer constituent of a bone replacement material, was determined using the ~'Cr release assay. An initial experiment used mouse L929 cells and a copolymer with an ambient pressure and temperature (APT) degassing time of 24 hr. Both copolymer and nylon control discs caused a significantly (P <~ 0.05) increased release of 5tCr from the cells when compared with discs that had not been EtO sterilized. In a second experiment using L929 cells, copolymer and nylon discs were EtO-sterilized and degassed (APT) for 1-14 days. After 24 hr in culture, discs degassed for up to 14 days caused significantly increased cytotoxicity. A third experiment used L929 cells and copolymer/demineralized bone matrix blocks with APT degassing times of 2, 4 and 6 wk, or vacuum degassing for 48 hr. A decrease in cytotoxicity was observed from 2 to 6 wk of degassing, but there was still a significant increase over the untreated discs at 2 and 4 wk and a non-significant increase at 6 wk. Vacuum degassing was comparable to 4 wk of APT. In the fourth experiment, vacuum degassing times of 1, 7 and 14 days were studied using both L929 and human fibroblast cells. Vacuum degassing for up to 14 days did not reduce the cytotoxicity to the levels observed for samples that had not been EtO sterilized. All four experiments demonstrated that EtO sterilization of the copolymer used in bone replacement materials caused increased in vitro cytotoxicity compared with non-sterilized copolymer, and that degassing either at APT for up to 6 wk or vacuum degassing for up to 14 days reduced, but did not eliminate, the cytotoxicity observed.