This paper reports the use of zonal ultracentrifuge techniques to conduct biophysical studies of rhinoviruses grown with WI-38 cells. Good clean-out of infectivity from rhinovirus harvests was obtained with the continuous-flow B-V and B-IX rotors. Use of the B-V rotor resulted in the successful concentration of rhinovirus infectivity and antigenicity. Additional purification was achieved by the combined use of continuous-flow centrifugation and isopycnic banding procedures. Two particle sizes were found to be associated with the virus-infected cell harvests. The infectious 22-nm particle banded in density ranges of 1.38 to 1.40 g/cm(3) in CsCl and 1.26 to 1.27 g/cm(3) in potassium citrate. The 8.0 nm capsomere was composed of 2.0 nm subunits and banded with a density of protein at 1.28 g/cm(3) in CsCl. Equivalent sedimentation coefficients of 155 or 185, depending on particle density in sucrose, were calculated from rate zonal experiments by use of the B-IV zonal rotor.