An investigation of persistent cell deaths of BHK21 suspension cells during subculturing resulted in the isolation of a viral agent. The agent was isolated from samples of dead cells, cell growth media and 2 batches of calf serum. It was established that the agent was associated with the use of certain batches of calf serum. The isolated virus was found to replicate effectively only in rapidly growing BHK cell cultures. In monolayers it caused the formation of large intranuclear inclusion bodies. The isolate was a strong haemagglutinin; it was stable to ether, chloroform, pH 3 and heating at 56 degrees C. It was shown to be a DNA-virus and by electron-microscopy it was evident as unenveloped spherical, small particles (21 nm in diameter). In sucrose density gradients the virus sedimented as 2 peaks at approximately 114S and 85-92S, and by caesium chloride equilibrium centrifugation, two main peaks at densities of 1.39 and 1.31 g/ml were evident with a minor peak at 1.35. The capsid of the complete virion consisted of 3 polypeptides. The agent has therefore been provisionally designated a member of the Parvoviridae family, genus parvovirus. Serologically it was found to be related to MVM. The isolated parvovirus was inactivated by 0.05 per cent peracetic acid and 0.05 per cent acetylethyleneimine.