The spin polarization of the disordered Fe-Co alloy was calculated using a tight-binding scheme, with single-site, full-orbital interactions treated self-consistently. Disorder is introduced by the use of the virtual-crystal approximation. Excellent agreement with the experimental spin polarization is obtained and the unusual shape of the Fe-Co curve on the Slater-Pauling plot is explained. The magnetization of the Co-rich alloys essentially depends on the number of available d holes, while the magnetization of the Fe-rich alloys is influenced by a relatively weak electron-electron interaction. The intersection of the two effects occurs at approximately 30 at.% Co and produces a maximum.