This report describes the isolation and partial characterisation of a cytomegalovirus (CMV) from the wild brown rat (Rattus norvegicus). The isolate was sensitive to ether and heat treatment at 56 degrees C/30 minutes, and had a characteristic herpes virus morphology. In rat embryo fibroblast cells, new virus was detectable by 18 hours after inoculation. The cytopathic effect consisted of a focal rounding of cells which developed to involve the entire monolayer. Inoculation of newborn rats produced mortality of 67-75 per cent which was reduced to 33-46 per cent by intra-peritoneal inoculation or tissue culture passage. Highest virus concentrations were found in the liver and spleen 14 days after inoculation. The virus was also pathogenic (to a lesser extent) to newborn BALB/c mice. It was related to the Osborn strain of mouse CMV by fluorescent-antibody testing but distinct by the serum-neutralisation test.