Large volumes of fluids from selected cell cultures producing ribonucleic acid tumor viruses were processed by a double sucrose density gradient procedure using zonal centrifuges. The primary recovery utilized a Model K continuous-flow zonal centrifuge at 9 to 11 liters/hr. The virus zone from the Model K gradient was further purified on a second gradient in a B-29 rotor. The resulting viral concentrates at 2 x 10(11) particles per ml showed high purity by electron microscopy and gel analysis and were useful reagents in biochemical and immunological studies of the reverse transcriptase enzyme, virus structure, complement fixation, and other virus properties. Similar recovery methods were applied to other tumor virus systems.