The linear dispersion relation for electromagnetic waves is investied for homogeneous plasmas normally found in the solar wind near 1 AU, including a secondary proton beam of varying strength and drift speed parallel to the ambient magnetic field. Three instabilities are found when the drift speed of the beam approaches the Alfv¿n speed CA: (1) field-aigned magnetosonic, (2) oblique magnetosonic, and (3) oblique Alfv¿n modes. Linear growth rates as a function of &bgr;=8&pgr;nTp/B0 2, drift speed, Te/Tp, proton and electron temperature anisotropies, and electron heat flux have been studied. Results show that beam-driven magnetosonic and Alfv¿n waves are likely to be important in the solar wind if the fraction of protons in the beam exceeds 5% and the drift speed exceeds ~0.8CA when &bgr;≲0.1 (or ~1.5CA when &bgr;>0.4).