Interleukin-10 (IL-10) is a T-helper type-2 (Th2) cytokine noted for its ability to suppress cytokine synthesis by T-helper type-1 (Th1) cells. IL-10 may play a role in pregnancy immunotolerance through the establishment of a Th2 cytokine bias at the maternal-fetal interface. This study examines the expression and production of IL-10 by normal and malignant human trophoblast. Term placental biopsies, cloned choriocarcinoma cell lines and isolated human trophoblast were utilized for the study of IL-10 expression. Choriocarcinoma cells (BeWo, JEG-3, JAR) were maintained in T-flask culture until confluence and then harvested by enzymatic dispersion. Purified term trophoblast were obtained by sequential trypsin/DNAse digests and CD9 immunoaffinity chromatography. Amplified IL-10 mRNA was detected by a reverse transcriptase polymerase chain reaction (RTPCR) technique. BeWo cells were maintained in artificial capillary culture (ACC) and conditioned media assayed for IL-10. Granulocyte macrophage-colony stimulating factor (GM-CSF; 1.0, 10.0 and 100.0 ng/ml) was added to the BeWo cultures to examine its effects on trophoblast IL-10 production. IL-10 determinations were performed using a human ELISA system. IL-10 mRNA was detected in each trophoblast cell type examined with the exception of the JEG-3 choriocarcinoma cell line. IL-10 protein was also detected (range 6-22 pg/ml) in BeWo media on days 8 to 11 of culture. When serum was reduced in the culture media, IL-10 levels fell below the sensitivity of the assay (5 pg/ml). Subsequent addition of GM-CSF stimulated BeWo IL-10 secretion in a dose-related fashion. These results support the concept IL-10 is expressed at the human maternal-fetal interface, and production of this important immunoregulatory molecule may be regulated, in part, by GM-CSF.