An electron microscopic study of somatic hybrids resulting from a cross between hamster melanoma cells harboring R-type particles and mouse fibroblasts harboring intracisternal A-type and (mostly) immature C-type particles was performed. The major findings are: (a) hybrid cells that retain a large fraction of both hamster and mouse chromosomes produce all three types of particles; (b) the loss of mouse chromosomes is followed by that of intracisternal A- and C-type particles; (c) the continued production of A- and C-type particles is dependent on the retention of different mouse chromosomes; (d) the mixed intracellular population of R- and A-particles comprises a small number of complex structures in which the two types of virions are enclosed within a common envelope.