A new strain of simian haemorrhagic fever (SHF) virus was isolated from chronically infected patas monkey no. 248 (P-248) in USU-104 cells. The P-248 isolate had the same size, morphology and cytoplasmic site of replication as the prototype LVR strain. However, the P-248 isolate caused a persistent infection without noticeable cytopathology in USU-104 cells rather than the strongly lytic infection produced by prototype LVR virus. The capacity of P-248 virus to produce a persistent, non-lytic infection of USU-104 cells was a very stable characteristic of the isolate. Extensive serial passage of this isolate through USU-104 cells (over 50 passages) and rhesus monkeys (six passages) failed to unmask virus with lytic properties for USU-104 cells. Culture medium from persistently infected cultures assayed in rhesus monkey peritoneal mononuclear phagocytes, where measurable cytopathology occurs, was found to contain about 10(5) to 10(6) TCID50/ml of cell-free P-248 virus. Immunolabelling techniques showed only a low percentage of infected cells in persistently infected cultures. The mechanism of persistence of the P-248 isolate in USU-104 cells has not been determined but evidence suggests it does not involve interferon or defective interfering particles.