We examined effects of 2-bromopropane (2-BP), a chlorofluorocarbon replacement, on mouse embryonic mutagenicity. 2-BP was administered to pregnant mice intraperitoneally (i.p.) (300, 600, 900, and 1800 mg/kg) during the early preimplantation period. On day 3 of gestation, micronuclei (MN) frequency and embryo cell number were determined. 2-BP induced a dose-related significant increase in MN frequency and a treatment-related decrease in embryo cell number. Furthermore, the cell numbers were significantly smaller in the MN-positive embryos by two-way ANOVA taking it into account an interactive effect between 2-BP dose and the presence or absence of MN. A simultaneous decrease in cell number and increase in MN frequency may reflect an embryonic developmental disadvantage resulting from maternal treatment with 2-BP. Further study is needed to establish how 2-BP contributes to postimplantation embryonic development. (C) 2001 Elsevier Science Inc. All rights reserved.