HVJ-induced fusion between liposomes and erythrocytes was examined as a model of cell-cell fusion and the following results were obtained: Liposome-liposome fusion seldom occurred in the presence of HVJ. Liposomes free of receptors for viral HN protein could efficiently fuse with erythrocytes when the concentration of liposomes and erythrocytes was high. Direct interaction between HVJ and liposomes should be negligible in the present system. A distinct lag phase (10 min) was observed before the liposome-erythrocyte fusion occurred appreciably. Efficiency of the liposome-erythrocyte fusion decreased linearly up to 1 h after stopping the virus-erythrocyte fusion. The former fusion was almost negligible after 1 h but could be restored by further addition of HVJ. Efficiency of the liposome-erythrocyte fusion was suppressed significantly when erythrocytes were pretreated with N-ethylmaleimide (N-EM), a membrane-permeable sulfhydryl agent, whereas rho-chloromercuriphenylsulfonic acid (PCMBS), a membrane-impermeable sulfhydryl agent, had no appreciable effect on the fusion efficiency. The study suggests possible involvement of some HVJ-induced 'activated state' of the erythrocyte membrane during the process inducing the liposome-erythrocyte fusion. The structure including the membrane skeleton system may be responsible for producing an 'activated state' in the erythrocyte membrane. Such an 'activated state' was induced 10 min after the viral infection to erythrocytes, and thereafter decayed gradually.