Summary Recombinant and parental poliovirus particles were indistinguishable by ratezonal and isopycnic sedimentation, and by u.v. inactivation. Sensitive selective procedures, applied to ts + recombinants to detect genetic segregation of one parent, failed to reveal any. Poliovirus genetic recombinant particles thus appear to be conventional virus particles; their significance for recombination mechanisms is discussed. Sensitivity to the growth inhibitor 2-(3-chloro-p-tolyl)-5-ethyl-1,3,4-oxadiazole is shown to depend on a product of the structural protein gene.