The nonlinear response of a high-temperature superconductor to an external ac magnetic field has been studied experimentally as a function of sample orientation relative to ac and dc external magnetic fields. The even harmonics of the response vanish in the case of a ceramic sample if the dc magnetic field is perpendicular to the ac field. This is not the case for a crystalline Bi₂ Sr₂ CaCu₂ O₈ sample where the amplitudes of even harmonics vary periodically with a sample rotated about the ac field. To explain this effect, a theoretical model taking into account the sample crystal structure is proposed.