We describe a method to prepare unilamellar fusogenic liposomes (FL) that can deliver their content directly into tissue cells and cancer cells of living animals as well as into cultured animal cells. The method consists of three important steps: 1) preparation of unilamellar liposomes encapsulating the molecules to be delivered: 2) preparation of the Sendai virus using fertilized chicken eggs; and 3) preparation of FLs by fusing UV-inactivated Sendai virus particles and naked liposomes. Purified FLs are stable at 4 degrees C for 3 weeks and can be stored indefinitely below -70 degrees C without loss of activity. The FL-mediated delivery is applicable to cells originating from various tissues, including those of humans, monkeys, cows, dogs, mice, rats, hamsters, and rabbits. However, human primary T cells primary B cells, and B cell lines are not susceptible to delivery. FLs are best suited to an examination of the biological activity of cytokines in the tissue cells of living animals. We also describe the possibility of developing FL-based hybrid vectors, which would mimic the functions of viruses and chromosomes.