In this paper, we report on submillimeter magneto-absorption studies of heavily doped (1 0 0) AlGaAs/GaAs heterojunctions. We found that the CR spectra consist of two branches with cyclotron-resonance effective masses of m=0.18 mo and 0.373 mo. These values are close to those predicted theoretically, and can be ascribed to the inversion asymmetry-induced spin splitting. In addition, we observed anticrossing features in the CR spectra. We discuss a possible origin of such CR line behavior as a coupling of light and heavy holes.