The role of membrane components in the interaction of cells with Newcastle disease virus (NDV) was studied using a ricin-resistant mutant of Chinese hamster ovary cells (CHO-15B), in which there is a deficiency in distal saccharides at the plasma membrane. Compared to the parental wild type, the mutant was shown to be 4- to 10-fold less sensitive to either fusion from without or fusion from within induced by NDV. The mutant and wild type were nearly indistinguishable with respect to other interactions with NDV. Viral attachment was investigated with 125I-labeled NDV and found to be comparable in both lines. Functionally equivalent amounts of hemagglutinin were produced, as measured by the fraction of cells positive for hemadsorption, or by the number of erythrocytes adsorbed per cell. No significant differences in the morphogenesis or yield of progeny virus were seen. The ability of the mutant to produce a fusion factor was measured by transfer of infected cells to uninfected monolayers. Infected CHO-15B cells were capable of inducing fusion normally in the indicator wild-type monolayers, but were incapable of inducing fusion in mutant monolayers. These results suggest that the insensitivity of the CHO-15B mutants to fusion may be due to inhibition of an early virus-cell interaction subsequent to viral attachment, whereas other events in infection appear to be unaffected by the cell surface mutation.