The electrokinetic surfaces of cells of the TA3 ascites tumour of the mouse and the M.Mel, 1 ascites tumour of the golden hamster have been studied for the past 3 years. It became evident that with regard to their electrophoretic mobility (E.P.M.), TA3 cells were stable and melanoma cells were not. Therefore, results discussed in this paper only refer to the melanoma ‘subline’ used for the present investigation. A comparison of the electrokinetic properties of the 2 tumours gives the following information: 1. TA3 and M.Mel, 1 cells have similar E.P.M.s: --1.03 and -1.O5 µm s -1 V -1 cm, respectively. 2. Treatment with 1.5% formaldehyde raises the E.P.M. of TA3 cells by 18% and of M.Mel. 1 cells by 11%. 3. Incubation with neuraminidase lowers the E.P.M. of TA3 cells by 46% and of M.Mel. 1 cells by 24%. 4. Consecutive treatment with formaldehyde/neuraminidase and with neuraminidase/formaldehyde produces identical pH-mobility profiles for TA3 and M.Mel. 1 cells. Further, the electrokinetic data suggest that in addition to anionogenic groups, cationogenic groups are present on the surfaces of both TA3 and M.Mel. 1 cells. Mitotic indices and E.P.M.s of standard washed and neuraminidase-treated M.Mel. 1 cells, examined on consecutive days after implantation, indicate a continuous loss of sialic acid from the electrokinetic surface during the progress of a transplantation passage. These results are discussed with regard to the significance attributed to ionogenic groups at the surface of normal and neoplastic cells by other authors.