HeLa cells infected with radioactive poliovirus type 2 were disrupted with ultrasonic treatment, followed by addition of a non-ionic detergent. Two types of virus particles were found to sediment at 80 to 90% the rate of native virus. The first of these appeared to be a complex of native virus particles and membrane components, since treatment with 0-2% SDS released infectious native particles. The second was non-infectious and its sedimentation rate was not greatly altered by SDS. One hour after infection this non-infectious particle was the major product of cell-mediated eclipse. We have confirmed that 10 to 30 mg-g/ml S-7, a substituted thiopyrimidine, blocks infection of cells by poliovirus in a specific manner. Analysis of cells infected with radioactive poliovirus type 2 in the presence of S-7 showed that the virus particles remained as the complex which can be disrupted with SDS. In addition to blocking cell-mediated eclipse, S-7 stabilizes poliovirus against heat inactivation in vitro at the same concentrations which block infection. This action resembles the effect of 10-2 M-glutathione, which is also known to block cell-mediated eclipse of poliovirus.