Publisher Summary This chapter describes the methods for the isolation and characterization of anucleate mammalian cells and the techniques for the fusion of anucleate and nucleated cells; and outlines certain of the potential experimental applications of anucleate mammalian cell systems. Cells formed by the fusion of anucleate and nucleated cells are not hybrid cells or heterokaryons in the conventionally accepted sense as two different genomes are not represented. The range of cell types susceptible to fusion by inactivated Sendai virus is extremely wide. Sendai virus for use in cell fusion experiments is grown in 10- to 11-day-old embryonated hens eggs as virus propagated in mammalian cell cultures has negligible cell fusion activity. The chorioallantoic membrane (CAM) of the egg is inoculated with 0.1 ml of Sendai virus, incubated at 36°C with the pointed end of the egg down for 3 days, and then maintained at 4°C overnight. Sendai virus preparations used for fusing cells are normally inactivated to avoid the problem of replication of infective virus within the fused cells and the accompanying virus-induced effects on cellular morphology and metabolism.