We calculate the third-order nonlinear absorption and dispersion (self-phase-modulation and cross-phase modulation) of a homogeneously broadened two-level molecular system with permanent dipole moments interacting with (1) an optical field of frequency, and (2) two optical fields of frequency 1 and 2, i.e., the susceptibilities (3)and (3)(-1;1,-2,2). Sharp features in the nonlinear absorption and dispersion appear whenever one- or two-photon resonance conditions are met. Only features corresponding to one-photon resonance arise if the difference between the permanent dipole moments of the levels, vanishes. The (3) spectrum has two features, one at frequency ba dominated by contributions proportional to ab4, where ab is the transition dipole moment of the two levels with transition frequency ba, and the other at frequency ba/2 dominated by contributions proportional to 2ab2. Sharp features in (3)(-1;2,-2,1) appear when 2 is near ba,1,1+ba, and 1-ba.