Sera from unselected and untreated patients with Hodgkin's disease (HD) were examined for antibodies to Epstein-Barr viral (EBV) capsid antigens (VCA). Delayed cutaneous hypersensitivity reactions were carried out with purified tuberculoprotein (PPD). Highly purified blood lymphocytes of the same patients were studied morphologically and classified for cell surface markers. Incorporation of 14C-thymidine was used as a measure of spontaneous DNA synthesis and DNA synthesis after exposure to different concentrations of three mitogens (PHA, Conconavalin A and pokeweed mitogen) and PPD. The distribution of EBV titres was in good agreement with previous reports. Most patients were lymphopenic, due to subnormal levels of T lymphocytes. The lymphocyte stimulation and skin tests showed different degrees of impairment in a considerable number of the patients. The results in 43 patients indicated that a relation exists between the immune defect and the anti-VCA titres. High serological anti-VCA reactivity was related to a poor cutaneous response to PPD, a decreased level of T lymphocytes in the blood and a depression of mitogen-induced DNA synthesis.