The effect of polylysine, HBr (PL), polyornithine, HBr (PO), protamine sulphate (PS) (low molecular weight basic polypeptides with 65% content of arginine, no tryptophan or tyrosine), β-alanine (βA) and heparin on the electrophoretic mobility (zeta potential—surface charge density) of saline-washed Ehrlich ascites tumour cells have been investigated. Only the lysine polypeptides (PO, PL) which have been reported to inhibit tumour transplantation/development change the net negative charge of the tumour cell membrane to zero or high positive values, presumably because of adsorption on the cell surface. Heparin inhibits this reaction and reverses the effect of the polypeptides, PO and PL, on the cells. When the tumour cells were treated with high doses of protamine sulphate or β-alanine, the net negative charge of the tumour cells was not altered to zero or positive values. Polyornithine changed the surface charge of human epidermoid cancer cells (HeP ≠ 2 in flask culture), human blood platelets and erythrocytes to zero and positive values. Since the PO-, PL-treated tumour cells inhibit tumour transplantation and also change the cell surface charge to zero or to positive values, it is suggested that the negative charge of the tumour cell surface may well play a role in its highly invasive properties.