Herpesvirus saimiri strain 11 and attenuated H. saimiri strain 11 proteins synthesized during the lytic cycle of virus replication were used for immunoprecipitation with various sera from natural (Saimiri sciureus) and experimental (Saguinus nigricollis, Saguinus fuscicollis, Aotus trivirgatus, New Zealand White rabbits) hosts. The analysis of the precipitates separated in sodium dodecyl sulfate:polyacrylamide gels revealed that in tumor-developing animals a specific set of viral polypeptides were precipitated, which were not precipitated by sera obtained from the natural host Saimiri sciureus. Using labeled proteins from H. saimiri 11 and its attenuated strain, respectively, a difference was shown after precipitation with a serum raised against infected cell proteins of H. saimiri 11.