During routine harvest of murine resident peritoneal cells for macrophage function assays the authors recently noted that mice showed a 3-4-fold spontaneous increase in number of peritoneal cells within 1 week of being placed in one of their animal rooms. While the mice appeared clinically normal, the collected macrophages had highly convoluted membranes, showed enhanced spontaneous tumor cell killing, and showed increased erythrophagocytosis. Histopathologic findings included mild peritonitis and occasional foci of individual hepatocyte necrosis. The results of routine murine serologic studies and bacterial cultures of the peritoneal cavity were negative. Immunosuppressed mice placed in this room showed severe hepatic necrosis within 4 days, and ultrastructural particles characteristic of corona virus could be demonstrated in the necrotic foci. Mouse hepatitis virus (MHV) was isolated from these livers. Untreated mice showed positive MHV titers, as detected by the enzyme-linked immunoabsorbent assay (ELISA) after 21 days in the room. This episode demonstrates that MHV have profound effects on macrophage parameters while causing few clinical signs or histopathologic alterations. Secondly, the complement fixation assay for MHV as included in routine viral screens appears relatively insensitive for detecting outbreaks of MHV.