Peripheral blood lymphocytes from 100 healthy subjects, 32 patients with active Hodgkin’s disease and 34 patients in clinical remission were studied by means of the rosette formation test. Rosette formation by the lymphocytes of patients in stages I, II and III of Hodgkin’s disease did not differ significantly from the normal controls. Patients in stage IV of the disease showed a significant decrease in the number of rosettes formed. The rosette formation from patients who had been in a clinical remission for more than 1 year did not differ significantly from the normal controls. The total white blood count, total lymphocyte count and percentage of rosette-forming cells were followed in six patients with Hodgkin’s disease receiving quadruple cytotoxic drug therapy. There was a significant fall in total white count during treatment and a slight but significant rise in the proportion of rosette-forming cells. It is concluded that a fall in the proportion of rosette-forming cells only occurred in advanced Hodgkin’s disease where the patient was generally ill. It was not a feature of early or of quite widespread disease. It seems unlikely that Hodgkin’s disease itself causes a specific depletion of rosette-forming cells.