Growth curves of BTV recombinant viruses used in this study were derived in cells infected at a MOI of 0.05 and testing for the presence of infectious virus in supernatants collected at 8, 24, 48, 72 and 96 h post-infection. Virus growth was also assessed in cells in the presence of 1000 antiviral units/ml (AVU/ml) of interferon Tau (IFNT) or universal type I interferon (UIFN). Recombinant ovine IFNT was kindly provided by Tom Spencer. IFNT was produced in Pichia pastoris and purified as described previously [35] . Universal type I Interferon (UIFN) was obtained from PBL InterferonSource. BFAE cells and CPT-Tert cells were treated with 1000 AVU of IFN 20 h prior infection with BTV recombinants at a MOI of 0.1 (BFAE and CPT-Tert), 0.01 or 0.001 (CPT-Tert). Two hours after infection, the medium was replaced and the cells maintained in the presence of either IFNT or UIFN at the original concentration. Cell supernatants were collected at 24, 48 and 72 h post-infection, centrifuged for 5 min at 500 g in order to pellet cell debris and virus infectivity was subsequently titrated by endpoint dilution analysis on BSR cells. Viral titers were calculated by the method of Reed & Muench and expressed as log 10 TCID 50 /ml [36] . Each experiment was performed two to three times, each time in duplicate, using different stocks for each virus.