We determined serial IgG antibody responses to cytomegalovirus (CMV)-encoded proteins in sera collected over a one-year interval from 14 subjects with CMV mononucleosis. Antigens from infected human fibroblasts included three components: Cytoplasmic, nuclear, and high-speed pellet. Antibody was detected by radioimmunoprecipitation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Twenty to 21 bands were observed with the cytoplasmic component, whereas 10 and 9, respectively, were seen with the nuclear and high-speed pellet antigens. The most intense reactions occurred with the higher-molecular-mass proteins (50–215 kDa) by using the cytoplasmic and high-speed antigens and with the more rapidly migrating proteins (`50 kDa) by using the nuclear antigen. The precipitin responses increased for three months or more after onset of symptoms with the nuclear and high-speed pellet antigens but peaked within one to two months with the cytoplasmic antigen. © 1987 by The University of Chicago.