Women with systemic lupus erythematosus (SLE) with or without antiphospholipid antibodies (APLA) suffer from a high rate of recurrent abortions perhaps as a result of specific antibodies that may damage the conceptus. We studied the effects of sera from women with SLE--with or without--APLA and recurrent abortions on 10.5-d-old rat embryos in culture. This was compared to the results of culture on sera from control women and on rat sera. In addition, we studied sera from women with SLE with or without APLA after treatment with low doses of aspirin and glucocorticosteroids. Seventy-three percent of embryos cultured in sera from women with SLE with or without APLA were malformed in comparison to only 10.2% in embryos cultured on control sera and 5.4% in embryos cultured on rat sera. The rate of anomalies was reduced to 37.5% in embryos cultured on sera from women with SLE with or without APLA after treatment, as in 6 of 13 sera, the treatment reduced or prevented the occurrence of embryonic anomalies. When sera were divided in to low- and high-risk sera, the effect of treatment was even more significant, as the average percentage of embryonic anomalies per serum was reduced from 81.7 to 44.7%. Specific ultrastructural changes were found in the yolk sacs of the embryos cultured on the sera from women with SLE with or without APLA by transmission electron microscopy and by scanning electron microscopy. It seems that the rat embryo culture system may be an important clinical diagnostic tool to identify women with recurrent abortions in whom the etiology may be immunologic rejection of the embryo and to assess the efficacy of various treatment modalities.