It was easy to observe the particles of barley stripe mosaic virus (BSMV) in parenchyma cells in the ultrathin sections prepared from infected barley leaves according to routine methods. But no particles of BSMV were seen in phloem xylem and plasmodesmata. The research work in which the unlabelled immunoperoxidase technique was employed indicated that there was always a large amount of BSMV proteins in sieve tubes of diseased leaves. BSMV proteins were often seen in young xylem and plasmodesmata, occasionally mature xylem was also filled with them. All these facts sugest that BSMV proteins are translocated through phloem, xylem and plasmodesmata in barley plants infected with BSMV in the same way in which normal metabolic materials are translocated in healthy barley plants.The possibility that BSMV is translocated within the diseasod barley plant in the form of nucleic acid is discussed. It was easy to observe the particles of barley stripe mosaic virus (BSMV) in parenchyma cells in the ultrathin sections prepared from infected barley leaves according to routine methods. But no particles of BSMV were seen in phloem xylem and plasmodesmata. the unlabelled immunoperoxidase technique was employed indicates that there was always a large amount of BSMV proteins in sieve tubes of diseased leaves. BSMV proteins were often seen in young xylem and plasmodesmata, occasionally mature xylem was also filled with them. All these facts sugest that BSMV proteins are translocated through phloem, xylem and plasmodesmata in barley plants infected with BSMV in the same way in which normal metabolic materials are translocated in healthy barley plants. The possibility that BSMV is translocated within the diseasod barley plant in the form of nucleic acid is discussed .