Synthesis and processing of the envelope proteins of influenza A virus (fowl plague virus) have been analysed in BHK, HeLa and L cells, in which the virus undergoes abortive replication and does not form virus particles, and in the productive chick embryo fibroblast system. In abortive infection, synthesis of the M protein is specifically inhibited. The extent of this defect varies depending on the host cell and the amount of virus particles formed closely reflects the amount of M synthesized. Cell fractionation experiments demonstrated that the haemagglutinin glycoprotein HA is synthesized in abortive as well as in productive cells at the rough endoplasmic reticulum, that it migrates via smooth internal membranes to the plasma membrane and that it is cleaved by proteolysis into fragments HA1 and HA2 in the course of migration. Immune electron microscopy using monospecific antibodies against haemagglutinin and neuraminidase showed that both glycoproteins are exposed at the cell surface. Thus, synthesis and processing of the virus glycoproteins does not depend on the formation of the M protein. However, the M protein appears to be necessary for budding and thus for particle formation.