An established method of investigating total surface charge on particulate materials has been applied to the detection of changes in surface charge on macrophages which are induced by supernatants from the interaction of sensitised lymphocytes and appropriate antigen. The method was studied as an alternative to the established but problematical macrophage-electrophoretic-mobility (M.E.M.) test for detecting such changes. This preliminary investigation suggests that the results from subjects with and without malignant disease differ significantly.