Through fractionation in the ultracentrifuge of partially purified concentrated poliovirus from monkey kidney tissue cultures, and by infectivity and serologic assay of the fractions so obtained, evidence has been obtained for the existence of four distinct viral particles. These have been designated ID+, NID+, NID- and NIC-, representing, respectively, infectious virus particles possessing immunologic D specificity, and a sedimentation coefficient of about 160 S, noninfectious particles of D specificity and 160 S, noninfectious particles of D specificity exhibiting a sedimentation coefficient of 80–90 S, and noninfectious particles of C specificity exhibiting a sedimentation coefficient of 80–90 S. The designation of C and D specificity refers to reactivity in complement fixation tests with suitable acute or convalescent phase sera, respectively.