Chinese yam necrotic mosaic virus (CYNMV) was purified from naturally infected chinese yam leaves by repeated differential centrifugation. The molecular weight of the capsid protein was determined as 38, 000 (38K) with SDS-polyacrylamide gel electrophoresis. The antiserum produced against purified CYNMV preparation had a titer of 1:4, 096 as determined by immunosorbent electron microscopy and was used for the detection of the 38K polypeptide from leaf homogenate by electro-blot immunoassay. The procedure was simplified to complete all the steps within 7 hours. The 38K polypeptide was readily detected from diseased leaf homogenate diluted up to 1/1, 000 but not from healthy leaf homogenate. Reaction with host plant proteins was reduced by diluting the antiserum up to 1/5, 000 without losing sensitivity to CYNMV capsid protein.