For entry experiments with transfected cells, BTV-1 (13 mg/ml in GMEM) was bound to the cells at 4uC for 40 minutes. Unbound virus was removed by washing and the cells placed at 37uC (in GMEM) to initiate entry. Where specified, inhibitors were added to the cells for 0.5 h before the virus binding step and remained present throughout the assay. All entry experiments were ended at the times indicated by the addition of 4% paraformaldehyde (PFM), with the exception of those involving transferrin which were ended using 4% PFM with 0.25% glutaraldehyde. Input virus was detected using the anti-VP5 antibody, PM10. For quantification of virus uptake, cells with cytoplasmic labelling for BTV were scored as positive. In all experiments, ,94% of the control cells showed virus uptake.