Pichinde and Machupo viruses labeled with 3H-uridine were concentrated and purified by different methods. The buoyant density of the viruses was for Pichinde 1.14-1.15 g/cm3 (urographimeter), 1.19 g/cm3 (sucrose), 1.25 g/cm3 (cesium chloride), for Machupo virus 1.25 g/cm3 (cesium chloride). Virion RNAs of Pichinde and Machupo viruses were divided in sucrose concentration gradient into 4 sedimentation classes: 7-10 S, 18 S, 20-22 S, and 28-30 S. Virion 28 S and 18 S RNAs cosedimented with ribosomal cellular RNAs. The RNAs isolated at late stages of infection from the infected cells labeled with 3H-uridine in the presence of actinomycin D (1 microgram/ml) contained two major sedimentation classes of virus-specific molecules: 20-22 S and 30-31 S. Like Pichinde virus, Machupo virus genome is supposed to contain two RNA fragments (30-31 S and 20-22 S) as well as admixtures of ribosomal RNAs.