Lymphocyte response to thymus (T)- and bursal (B)-cell stimulation was studied in 26 patients with a variety of cancers. It was found that a T-cell block was present in ten of 13 patients with an unfavorable clinical course, whereas in patients undergoing a favorable course, only three of 11 were blocked. The blocking substance was in the serum and could be washed free from the cells, which then responded normally. The blocking substance was associated with a rise in α2-globulin, as noted by disk-gel electrophoresis. There was no correlation of B-cell stimulation with the patients' course or T-cell response.