A bovine trophoblast interferon (IFN-τ) gene promoter sequence (− 450 to +26 bp relative to the transcription start site) led to expression of reporter gene (CAT) constructs transfected into L929 (murine fibroblast) or JAR (human choriocarcinoma) cells. Expression depended on the presence of an exogenous (SV40) enhancer. Poly(I)(C) activated endogenous IFN production in L929 and JAR cells but had no consistent effect on CAT expression. Similar results were obtained in L929 cells with inactivated Newcastle disease virus. There was no 'priming' effect of exogenous Type I IFN. Deletion mutants revealed sites exerting negative control on expression between −338 and −247 bp, and between −150 and −71 bp; these regions contained sequences resembling previously identified negative regulatory domains. In the absence of viral inducibility it is proposed that negative regulation contributes towards the stringent control of expression characteristic of IFN-τ genes.