The influence of adsorbed polypeptides on electrofusion of histiocytic U937 and myeloidic K562 cells was extended by the corresponding monomeric amino acids in 0.3 M mannitol solution at pH 6.6 and other pH values. In all cases the monomers showed a similar behavior to the polymers, namely, an increase in the relative fusion yield Fr, or, in the case of sulfur-containing compounds, a decrease in Fr at higher concentrations. These electrostatic effects - dominated by the isoelectric points pI and the pH of the medium - are modified by mechanical interventions (e.g., bridging), because the Fr value also depends on the molecular mass (Mm) of the polypeptides. In fact, it rises with increasing Mm. These results demonstrate the fundamental interactions among cell membranes and proteins, polypeptides, and amino acids, which are responsible for essential life processes.