Peripheral blood T and B lymphocytes were quantitated in 42 patients with untreated Hodgkin's disease and the results compared with the response to phytohemagglutinin (PHA) stimulation and delayed hypersensitivity skin testing. T lymphocytes were identified by an in vitro cytotoxicity assay employing a specific anti-T-cell serum and by spontaneous rosette formation with sheep erythrocytes (E rosettes). The percentage of T cells in the patients was similar to that of normal subjects as judged by the cytotoxicity assay (65 to 90%). In addition, absolute T-lymphocyte counts were normal in 63% of the patients and were generally reduced only in those with lymphopenia. The percentage of T lymphocytes determined by the E-rosette assay was similar to that determined by the cytotoxicity assay in normal controls, but was significantly lower than that determined by the cytotoxicity assay in the patients. Moreover, the decreased response to PHA stimulation in the patients was directly correlated with the decrease in E-rosette formation. These findings suggest that T lymphocytes in the peripheral blood are not generally diminished in untreated Hodgkin's disease. However, a proportion of these cells exhibits altered surface interactions that may account for some aspects of their impaired immunologic function.