Analysis of spontaneous lymphocyte transformation, as measured by 3H-thymidine incorporation by low density lymphocytes, before and after therapy was performed in 28 patients with Hodgkin's disease. Before treatment increased 3H-thymidine uptake was found in 24 out of 28 patients. Following therapy the spontaneous lymphocyte transformation of the patients in clinical remission returned to the normal range, whereas none of the patients with residual disease achieved these levels. The overall correlation between the uptake of the DNA precursor by low density, lymphocytes and disease activity after treatment was excellent.