Phenotypically distinct rat trophoblast cell lines, the Rcho-1 and R8RP.3 cells, were compared for their responses to cytokines known to induce major histocompatibility (MHC) class I antigens, tumour necrosis factor (TNF), transforming growth factor-β (TGF-β), and interferon-γ (IFN-γ). Cell enzyme immunosorbent assays and flow cytometry experiments showed that only IFN-γ could induce RT1 class I antigens on the Rcho-1 cells. Non-adherent cells were slightly less responsive than adherent, giant cell-like Rcho-1 cells. By contrast, RT1 class I antigens on the R8RP.3 cells were induced by both TGF-β1 and IFN-γ. The cytokines also had different effects on mitochondrial enzyme activity in the two lines. TNF and TGF-β1 mRNAs were demonstrated in both lines by using Northern blot hybridization. Rcho-1 but not R8RP.3 cells contained two TNF messages (≈2.2, 1.9 kb). Steady state levels of transcripts from the TNF gene, and, to a lesser extent, the TGF-β1 gene, were increased in cultures of Rcho-1 cells that contained high proportions of giant cells. Thus, phenotypically distinct rat trophoblast cell lines do not respond identically to TNF, TGF-β1 or IFN-γ, transcription of cytokine genes does not prevent the cells from responding to paracrine cytokine signals, and the cells contain novel TNF transcripts that might be important in cell maturation or differentiation.