Determination of electrophoretic mobility of lymphocytes in 70 cancer patients shows a significant decrease of the fast moving cells. It seems to be a relation between the percentage or absolute number of the rapid lymphocytes and dissemination and prognosis of the disease. This decrease is in agreement with the variations of the T lymphocyte population described in cancer patients with other methods. On the contrary, although there is reduction of total blood lymphocytes, the number of slow moving cells is not reduced. The changes observed in mean migration speed of fast and slow migrating lymphocytes could indicate more selective alterations in some cell subpopulations.