In a longitudinal study we examined by immunoblotting (IB) the development and the evolution of the humoral immune response against individual cytomegalovirus (CMV) structural polypeptides in a total of 80 serum samples from 13 renal transplant recipients showing serological evidence of CMV infection and five renal transplant recipients with an anti-CMV antibody level unchanged over the observation period. The results showed that the IB reactivity at the time of transplantation may be a good index of the host's humoral immune status against CMV; by using this procedure it is possible to identify a seroconversion by the detection of antibodies reacting with some intermediate molecular weight proteins in sera examined at high dilution. Furthermore, IB is a very sensitive procedure also for IgM detection as it anticipates the positivity of the enzyme immune assay for IgM.