Acidic pH is required for productive reovirus infection of murine L929 fibroblasts [10, 27] , in which the aciddependent proteases Cat L and Cat B mediate uncoating [21, 23] . Serine proteases, including NE, and metalloproteases function over a broader pH range. Therefore, to gain insight into the nature of the protease(s) that can promote reovirus uncoating in U937 cells, we investigated the requirements for acidic pH. L929 and U937 cells were left untreated or pre-treated with E64 in the presence or absence of bafilomycin A1 (Baf) or NH 4 Cl. These latter agents raise vesicular pH by blocking the vacuolar H + -ATPase pump or by acting as a weak base, respectively [48] [49] [50] . After pre-treatment, cells were infected with Lang virions at an MOI of 3 and viral yields were determined at 3 days post infection (d p.i.). A representative experiment is shown in Fig. 2 . Treatment with either Baf or NH 4 Cl did not inhibit viral replication in U937 cells; yields reached 2.9 and 2.7 logs, respectively. Furthermore, these agents had little effect on viral replication in U937 cells even when the cells were also treated with E64 to inhibit cysteine protease activity. In contrast, Baf or NH 4 Cl alone completely blocked reovirus replication in L929 cells, consistent with the requirement for Cat L/B-mediated σ3 removal in these cells. Given that reovirus uncoating is an essential step in the viral life cycle [10] , these findings revealed that a non-cysteine protease that functions at neutral pH can facilitate this step in U937 cells.