Four murine tissue culture cell strains, which originated by cloning from one common cell of subcutaneous connective tissue origin, were examined for the presence of virus by electron microscopy and complement fixation techniques. The relative distribution of C particles and intracisternal A particles was determined. Thereafter, the characteristics of and relationship between A- and C-type particles were investigated. Cell extracts were passaged onto virus-free Swiss and C3Hf mouse embryo tissue cultures; CF and EM tests were again made to investigate the infective capacities of the various particle types. Although C particles were found in passage cultures exposed to extracts from the C particle-bearing strains, no A particles were found in any passage culture. These results indicate that the intracisternal A particle was neither infective nor developmentally associated with the C particle. A positive CF test was correlated with the presence of morphologically detectable C particles and was independent of the concentration of A particles.