Ovine progressive pneumonia (OPP), a chronic debilitating disease of adult sheep, develops insidiously over a long incubation period. The infection is widespread and may produce a fatal multisystemic lymphoproliferative disease. There is evidence that OPP virus infection is common in western and midwestern sheep flocks. A survey of cull ewes from western states revealed serological prevalence rates of up to 67 %. In Idaho, infection rates within range flocks were 51-90 %. A recent serological survey of the Iowa State University sheep flock indicated that 49 % of the ewes have opp virus antibodies and carry the virus.