LLC-MK<sub>2</sub> cells are susceptible to SV40(MK<sub>2</sub>) virus which produce a nonlytic infection with small virus yield. A very small input multiplicity of virus produces marked effects. After an early stage of protein synthesis inhibition, virus activity is associated with the formation of polyribosomes, especially those of fractions 7 (tetraribosomes), 5 (pentamers), and 1 (polysomes with 7 and more ribosomal units). This is followed by changes in ribosomal activity, leading to a generalized inhibition of protein synthesis through the formation of inactive polyribosomes by subunit binding, which may be due to alteration of the dissociation factors, or to the modification of the ribosomal proteins. Inhibition of protein synthesis is responsible for the low virus yield.