Twenty patients, cadaveric renal transplant recipients, were retrospectively analysed for serum levels of deoxythymidine kinase. Special reference was made to the thymidine kinase level in relation to rejection, and viral infection. Seven of the patients experienced clinically suspected cytomegalovirus infection. All these patients had elevated levels of serum thymidine kinase during the period of clinical disease. Usually the thymidine kinase level parallelled the severity of the disease. All patients with irreversible rejection had increased levels of serum thymidine kinase, but normally not as high, as seen in patients with severe cytomegalovirus infection. There was also some correlation between clinically suspected rejection, that lead to rejection treatment, and moderate increase in thymidine kinase. However, not all rejection episodes were accompanied by a thymidine kinase increase. Serum thymidine kinase was analysed and compared in patients with self-healing cytomegalovirus disease, and those treated with phosphonoformate. A rapid decline in thymidine kinase level was found in connection with successful antiviral therapy, when compared to the decline in untreated patients. Some bone marrow transplanted patients were also included in this analysis.