The present work investigated antigenic variations of the most immunogenic polypeptides of Cytomegalovirus (CMV), namely two nonstructural polypeptides of MW 72 Kd and 52 Kd, and 5 structural proteins of MW 150, 65, 58, 38 and 28 Kd, in several low-passage CMV isolates from urine of infected children. The polypeptides showing the greatest variations were the non structural p72 and p52, and the structural proteins p65 and p38. Furthermore the reactivity of the children's sera was studied with regard to the antigenic proteins of the homologous virus strain. We failed to recognize any reactivity pattern typical of an acute CMV infection and the results obtained indicate that CMV-infected children may have a very low and variable IgG and IgM response, p150 and p65 being the two most often recognized antigens.