A correlation between the proton core plasma beta and the proton beam drift speed in units of the local Alfv¿n speed has been found in high-speed solar wind with the Helios 2 spacecraft plasma data obtained in 1976. This relation reads vd/vA = (2.16 ¿ 0.03) ¿$parallel$c(0.281¿0.008) for the range ¿$parallel$c = 0.1 to 0.6, where ¿$parallel$c is the proton core plasma beta determined from the proton core thermal velocity component parallel to the magnetic field, vd is the proton beam drift speed relative to the core, and vA is the local Alfv¿n speed. This relation places a tight constraint on theoretical models which describe the formation of the proton beam in the fast solar wind. It is also found that most of the observed proton beam distributions are stable with respect to the electromagnetic proton instability.