Long periods can be required to determine the color stability of acrylic resins. The concentration of propagating radicals with unpigmented experimental resins (UER) and initiator solution (IS) of two initiator systems (binary and ternary) and five products were measured using electron spin resonance (ESR). The radical concentration was estimated from the area under the integrated ESR spectrum. The discoloration was measured using a color computer. Higher content of benzoylperoxide led to higher radical concentrations. The radical concentration for the ternary system increased slightly with time, but were lower than those of the binary system. Significant positive correlations were found between the radical concentration and ΔE*ab for UER (r=0.900, p<0.01), IS (r=0.953, p<0.001) and the products (r=0.808, p<0.05). It was suggested that the color stability of these resins was predictable by measuring the radical concentration using ESR.