Using radioimmunoassay followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, we compared serial IgG precipitin antibody responses to cytomegalovirus (CMV) proteins in three groups of 29 pregnant women who had primary CMV infection. Five women had CMV mononucleosis, and four of them infected their fetuses. Twenty-four women had subclinical infection, and eight infected their fetuses. There were no qualitative differences in the precipitin responses against the virus-encoded proteins in three different infected cellular antigens (cytoplasmic, nuclear, and high-speed pellet) between these three groups of women. There was also no qualitative difference in responses whether the infection was clinically apparent or subclinical. Quantitation by densitometer, however, revealed that women with mononucleosis and those with subclinical infection who infected their fetuses had a more intense and prolonged antibody response than did women with subclinical infection who failed to transmit CMV in utero. © 1988 by The University of Chicago.