Non-infectious virus particles produced by influenza virus (classical fowl plague)-infected Ehrlich ascitic carcinoma cells have the same morphology, size and sedimentation rate as the standard virions. Their main difference from the allantoic virus is their extreme fragility. They remain intact upon a short-term centrifugation in sucrose solutions but desintegrate upon prolonged centrifugation. In isopicnic fractionation they are detected in two forms, with a density of 1.23 g/ml retaining the hemagglutinating but not the neuraminidase activity and with a density of 1.27 g/ml deprived of both hemagglutinating and neuraminidase activity. In the electron microscopic examination the 1.23 g/ml structure appears as virus particles with defective areas in the surface spikes layer whereas the 1.27 g/ml structure has no spike layer at all. The protein analysis by polyacryl amide gel electrophoresis revealed a sharply reduced amount of the membrane protein in the ascitic virus. A hypothesis is suggested according to which the reduced amount of the membrane protein is the cause of the unusual fragility of ascitic virus particle membranes as a result of which they readily lose glycoproteins which, in its turn, leads to their reduced infectious activity.