Four different types of envelope of Sendai virus or subviral components, that is, infectious and non-infectious virions, reassembled envelope particles (REP), and Tween-ether-treated envelope fragments (TE), were studied comparatively for membrane interactions with chicken erythrocytes by freeze-fracture electron microscopy, specifically for membrane alteration by envelope fusion. The freeze-fracture replicas of the attachment of the four envelopes in the cold exhibited a common pattern of impressions with attached envelopes, although the fracture plane traversed from erythrocyte to envelope at the periphery of the contact areas of three of the envelopes but not of TE, where the fracture plane mostly cut only through erythrocyte membranes impressed with TE. The freeze-fracture replicas of the four envelopes reacting with erythrocytes after a short incubation period at 37 C exhibited distinctive features: infectious virions and REP displayed evidence of envelope fusion, but non-infectious virions and TE showed a particular pattern of envelope association without fusion. Our data demonstrate that the pattern specific for envelope fusion is the formation of a continuous membrane from envelope to cell membrane in a cross fracture of an erythrocyte.