An agar gel immunodiffusion test was developed to detect precipitating antibody against ovine progressive pneumonia (OPP) virus. The test was conducted in plastic petri dishes containing 6 ml of 1% purified agar in tris buffer and 8% sodium chloride. Wells for serum and antigen were 8 mm in diameter and were cut in a hexagonal pattern 3 cm from a central well. Tests were read at 24 and 48 hours. Soluble antigen for the test consisted of concentrated nutrient medium removed every 2 weeks from a cell culture persistently infected with isolate WLC 1 of OPP virus. Specificity of results was verified by testing serums from experimentally exposed sheep and appropriate controls. Two lines of precipitate formed with some serums from experimentally inoculated sheep. Serums taken soon after exposure of sheep to the virus and those taken 3 to 4 years after exposure frequently formed only 1 line of precipitate. Of 37 lambs inoculated with OPP virus, 25% of those tested were positive by postinoculation (PI) month 1, 79% of those tested were positive by PI month 3, and all of those tested were positive by PI month 6. The test appears adequate to detect exposure of sheep to OPP virus.