To validate and extend these results we visualized the reduction of the number of infected cells by immunoperoxidase staining using an antibody against NP (Fig. 3) . A number of different cells of mammalian and avian origin were infected for 2 hours at an MOI of 1 in PBS with or without serum. After 2 hours the inoculum was replaced by growth medium containing 10% FCS and 10 nM BafA1 and the expression of NP was examined after 14 hours later. After incubation in PBS, staining was completely prohibited by the presence of 80 mM dynasore whereas in the presence of serum dynasore had no effect. A serum-inducible, DYNA-IND route of entry was thus functional in all five cell lines, including the human epithelial airway carcinoma cell line A549.