Lymphocytes from MC sarcoma-bearing mice were exposed to a soluble tumor antigen. The lymphokine-containing supernatant thus obtained was fractionated on Sephadex G-75. Fractions were tested for their activity to reduce the surface charge of indicator macrophages by cell electrophoresis technique. When un-separated spleen or lymph node cells were used the charge-reducing activity of the supernatant was found in three regions according to molecular weights of about 100,000, 60,000 – 35,000 and 13,000 Daltons. The appearance of activity was found to be time-dependent. As indicated by tests with separated lymphocytes both B and T cells appear to release charge-reducing activity.