When mouse mammary carcinoma (FM3A) cells were treated with egg agglutinin of Rana catesbiana for 15 min at 25 degrees C, percent total particle number of both cell aggregates and single cells was in direct proportion to the cell electrophoretic mobility. Phytohemagglutinin P mediated agglutination proceeded with biphasic kinetics: the higher the concentration of phytohemagglutinin P the shorter was the lag period between the first and second stages of agglutination. In protamine-mediated agglutination, the percent total particle number was reduced at low concentrations, while the electrophoretic mobility reduced only at high concentrations. Agglutinating and cytotoxic activities of these three reagents were in an intimate relation: the higher the agglutinating activity, the greater was their cytotoxic activity.