Ehrlich ascitic carcinoma cells infected with influenza virus, WSN strain, produced noninfectious virus particles differing by some properties from standard virions formed in chick embryo. The ascitic particles were unusually fragile and lost hemagglutinins during purification and storage. They were found in larger or smaller aggregates. In the electron microscope they appeared as spikeless particles of heterogeneous size. Polyacrylamide gel electrophoretic analysis of polypeptides revealed markedly decreased contents of hemagglutinins and M-protein. It is suggested that the decreased amount of M-protein results in the fragility of ascitic virus particles and loss of hemagglutinins which, in its turn, leads to the loss of infectivity. Ribonucleoproteins (RNP) synthesized in the cytoplasm and nuclei of Ehrlich ascitic carcinoma cells differed in their biophysical properties from RNP formed in the permissive system (chick fibroblast culture). They sedimented slower than RNP from chick fibroblasts, and their buoyant density was 1.37--1.40 g/ml instead of 1.34 g/ml, i. e. the density typical of influenza virus nucleocapsid.