A study was undertaken to examine polyoma DNA-protein complexes. A biophysical characterization of the complexes was made, and the proteins found in such complexes were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A comparison was made between a 52S nucleoprotein complex isolated from nuclei of 26-h polyoma-infected cells and a 28S virion core complex ejected out of mature virus particles. It was found that both complexes were reduced to a 20S viral DNA component plus free protein after incubation in 1 M NaCl or Sarkosyl. Treatment of the complexes with either Pronase or 0.5 M NaCl resulted in only partial removal of proteins from the viral DNA. After fixation in formaldehyde, the 52S nucleoprotein complex had a buoyant density of 1.45 g/cm(3), and the virion core complex had a buoyant density of 1.59 g/cm(3). Sodium dodecyl sulfate-polyacrylamide gel profiles of purified polyoma virion proteins, used as a reference marker, demonstrated three capsid proteins, V1 to V3, as well as four histones, V4 to V7, which constituted about 7% of the total virion protein. Electrophoretic analysis of the proteins comprising the 52S nucleoprotein complex revealed that the same seven proteins present in the mature virion were also found in this complex. However, the ratios of the proteins in the complex were quite different from that of the mature virion, with the four histones comprising 48% of the total complex protein. A pulse-chase experiment of the nucleoprotein complex demonstrated that the 26-h complex was chased into mature virions.