Amphoteric random copolymers P(AMPS/APTAC50)(x), where x = 41, 89, and 117, composed of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and 3-acrylamidopropyltrimethylammonium chloride (APTAC) were prepared via reversible addition fragmentation chain transfer radical polymerization. P(AMPS/APTACSO)(x) can dissolve in pure water to form small interpolymer aggregates. In aqueous solutions of NaCl, P(AMPS/APTACSO)(x) can dissolve in the unimer state. Amphoteric random copolymer P(AMPS/APTAC50)(c) with high molecular weight was prepared via conventional free-radical polymerization. Although P(AMPS/APTACSO)(c) cannot dissolve in pure water, it can dissolve in aqueous solutions of NaCl. In amphoteric random copolymers with high molecular weight, the possibility of continuous sequences of monomers with the same charge may increase, which may cause strong interactions between polymer chains. When fetal bovine serum (FBS) and polyelectrolytes were mixed in phosphate-buffered saline, the hydrodynamic radius and light-scattering intensity increased. There was no interaction between P(AMPS/APTAC50)(x) and FBS because corresponding increases could not be observed.