In the present study, the aether extracts of castorbean seed (AEC), which possessed antifertility activity and contraceptive efficacy in female and male rodents, were evaluated for their in vitro inhibitory activity in the primary cultured rat decidual stromal cells (DSC). A bioassay-guided fractionation technique was used to separate active components from crude extracts. A colorless crystal showed a significant inhibitory activity (IC(50) = 63.84 +/- 3.04 microg mL(-1), r = +0.9478). The chemical analysis of the colorless crystal by capillary gas chromatography-mass spectrometry (GC-MS) revealed that the colorless crystal was a mixture of five components: four phytosterols which were ergost-5-en-3-ol (6.10%), stigmasterol (35.80%), gamma-sitosterol (44.77%), and fucosterol (8.40%); and one probucol analog (4.93%). It was presumed that gamma-sitosterol may be the main component contributing to inhibit the viability of DSC.