Summary: Respiratory syncytial virus was studied with respect to the effect of type of cell and composition of nutrient fluid on cytopathic manifestations and multiplication in tissue culture. Production of syncytium was striking in HEp-2 cells, minimal in KB and HeLa cells; rounding was the characteristic response of DMB cells. In comparison with a maintenance fluid containing Eagle's minimal essential medium, a fluid utilizing Scherer's solution limited the formation of syncytium and delayed the evolution of cytopathic effect, particularly in HEp-2 cells. The presence or absence of syncytium had little effect on the total yield of infectious virus. The latent period of the initial multiplication cycle was 12 to 14 hr, and was followed by rapid growth and release of virus. A method for cell disruption which released cell-associated virus with little inactivation was utilized to demonstrate that nearly 50% of virus is still cell-associated even when the cytopathic effect is marked. Some virus was inactivated by a single freeze-thaw cycle, but aliquots stored at -70°C maintained stable and adequate levels of infectious virus for at least 6 months.