An in vitro test for teratogens has been used successfully for more than 3 years. The method involves exposing undifferentiated rat embryo midbrain and limb cells to test compounds and observing the effect on subsequent cell differentiation. Experience of using the test has confirmed the accuracy of prediction (greater than 90%) suggested by a blind trial. The test has been used at the early stages of pharmacological evaluation in the selection of non-teratogenic pharmaceuticals and 250 compounds are tested on average each year. Maternal metabolism is modelled by the inclusion of Aroclor 1254-induced rat liver homogenate plus cofactors (S-9 mix). The concentrations of S-9 mix (50-100 microliters/ml culture medium) conventionally used in the Ames bacterial mutagenicity test are toxic to rat embryo cells, but greatly reduced concentrations (3-5 microliters/ml) are not toxic but are still able to activate pro-teratogens such as cyclophosphamide. However, most potentially teratogenic compounds tested are toxic in the absence of active preparations of drug-metabolizing enzymes. The conclusion that most teratogens are direct acting may be premature, since evidence has been found for drug-metabolizing activity in the embryo cells themselves.