A previously proposed method based on soap-free emulsion polymerization with an amphoteric initiator for producing micrometer-sized polystyrene particles was extended to application with methyl methacrylate (MMA). The aggregation and dispersion stability of polymer particles, which have ionizable groups arising from initiator radicals, can be controlled by adjusting the pH of the reaction system accompanied with the addition of ionic monomer. Polymerizations were carried out with 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate amphoteric initiator, NH3/NH4Cl pH buffer, and sodium p-styrenesulfonate anionic monomer (NaSS) in ranges of MMA concentration (0.58-2.32 kmol/M-3 H2O) and NH3 concentration (2.5-20 mol/m(3) H2O) at fixed concentrations of 5 mol/m(3) H2O initiator, 10 mol/m(3) H2O NH4Cl, and 1 mol/m(3) H2O NaSS at 65 degreesC. The addition of NaSS during the polymerization could improve stability in dispersion of particles, which coagulated in the absence of NaSS after the disappearance of monomer drops. An increase in the monomer concentration in the present method could enlarge the particle size without lowering the monodispersity of the particle size distribution. On the other hand, an increase in NH3 concentration decreased the particle size. The highest monodispersity of particle sizes was obtained at a NH3 concentration of 5 mol/m(3) H2O, which gave an average size of 1.5 mum and a coefficient of variation of particle size distribution of 2.2% that was much smaller than the standard criterion for monodispersity, 10%.