The normal growth of rat embryos cultured in human serum supplemented with glucose and 10% rat serum is of potential use in the assessment of teratogenic risk in man. Before this technique can be widely applied, it is necessary to know whether the stages of the menstrual cycle at which a serum sample is tested materially affect the results obtained. Moreover, in order to achieve reproducible conditions we have found it necessary to add a minimal amount of rat serum to the human serum used for culture, but unnecessarily high levels of rat serum supplementation could over-compensate for any growth factor deficiency in human serum. Here we report that culture of rat conceptuses gives similar results irrespective of whether human first, second, or third trimester pregnancy serum, postnatal serum, or serum at various stages of the menstrual cycle is used. We also report that addition of 2% rat serum supplement is sufficient to achieve reproducible rat embryonic growth and differentiation in human serum.