After incubation with an encephalitogenic factor from human (HEF) or rat (REF) brain, lymphocytes of Fischer 344 rats bearing a spontaneous mammary adenocarcinoma produced a soluble substance which reduced the mobility of tanned sheep red blood cells in the electrophoretic mobility test (EMT). For studying the kinetics of this lymphocyte response, 6 X 10(5) tumor cells were injected into the hind footpad. In correlation with time and tumor size, one was able to influence the appearance of metastases by amputation of the leg. As early as 16 hours after inoculation of tumor cells, sensitivity of lymphocytes against HEF and a KCl-extract of the tumor could be shown in the EMT. It decreased on days 2 and 5, but was still seen until the day of amputation. Rats without metastases showed sensitivity up to four weeks after amputation and then returned to normal levels. Rats with metastases showed sensitivity until death at about seven weeks later. With the use of Amicon membranes, Sephadex G-50, and ion-exchange chromatography, a protein could be isolated from human basic myelin extract with a molecular weight of about 16,000-20,000 daltons. It had no direct influence on the EIC by itself, but after incubation with lymphocytes from tumor-bearing rats it evoked the production of a slowing substance. Using Sephadex G-100, the slowing substance appeared in the region in front of BSA indicating a molecular weight of greater than 80,000 daltons. It was heat-stable for 30 min at 56 degrees C and was sensitive to trypsin.