We report a comprehensive infrared magnetospectroscopy study of a CeRu4Sb12 compound revealing quasiparticles with a heavy effective mass m(*), with a detailed analysis of optical constants in fields up to 17 T. We find that the applied magnetic field strongly affects the low-energy excitations in the system. In particular, the magnitude of m*similar or equal to 70 m(b) (m(b) is the quasiparticle band mass) at 10 K is suppressed by as much as 25% at 17 T. This effect is in quantitative agreement with the mean-field solution of the periodic Anderson model augmented with a Zeeman term.