Objective: Using a mouse embryo bioassay, we examined the embryotoxic properties of serum obtained from eight women who had ingested cocaine base paste (CBP) for a period of 1-3 years. Study design: Two-cell stage mouse embryos were cultured in the presence of serum extracted from eight CBP-smoking or control women. After 48 h, embryos were evaluated to determine their developmental stage and then processed by Tarkoswki's technique to determine the proportion of embryos with micronuclei, in order to establish the number of micronuclei/embryo. Results: Serum from CBP patients diminished the percentage of embryos progressing to the compacted morula stage, while increasing the proportion of embryos with micronuclei and the corresponding micronuclei/embryo ratio. Conclusion: These findings provide the first experimental evidence of embryotoxic compounds in the serum of CBP-smoking women. This study highlights the reproductive risk of chronically ingested CBP and demonstrates the need for further studies. © 2007 Elsevier Ireland Ltd. All rights reserved.