534 serum samples from 73 renal transplant recipients, 41 haemodialysis patients, and 99 blood and organ donors were examined serologically for antibodies against Cytomegalo, H. simplex (types 1 and 2), Varicella-zoster, Epstein-Barr, Adeno, Influenza, Parainfluenza, Respiratory syncytial, Measles, Picorna- and human Polyoma- Viruses. Serum specimens were stored in the lyophilized state until examined thus enabling a simultaneous testing of all samples belonging to one patient. All antigens, complement, and control antisera were prepared, lyophilized, and standardized in this laboratory. This has enabled the use of single batches of any preparation throughout the study. Serologic results with antigens of the Herpesvirus group (CMV, HSV and VZV) compared favourably with previous results showing that infections with these agents, especially with CMV, can frequently be encountered among transplant recipients. Our results have indicated a moderately increased incidence of infections with some Herpesviruses in haemodialysis patients as well. Infections with VZV, for instance, were as frequently demonstrated in these patients, as in transplant patients although the former received no immunosuppressive therapy. Serologic results with non-Herpesvirus antigens indicated an increased incidence of infections with Polyomavirus, Myxoviruses (Influenza, Parainfluenza and RS) and Picornaviruses among transplant recipients. The incidence of acute infections with RS virus among adults was unusually high and there is no evidence so far to indicate such a high frequency of RS infections in any other group of adults. We were unable to demonstrate acute infections with non-Herpesviruses among haemodialysis patients, even though most of the patients were followed over a period of more than 2 years. Virus isolation attempts were performed with samples of urines and biopsy or autopsy samples. 23 out of 28 cytopathic agents recovered from urines, throat-swabs and/or from organs of transplant recipients were identified as CMV. Two HSV type 1, 1 HSV type 2, and 2 Coxsackie B type 3 viruses were also isolated. No viruses were isolated from a series of 31 kidneys randomly selected among autopsy cases.