Electronic intrasubband excitations are studied by means of resonant inelastic light scattering by a two-dimensional electron gas. The different effect of sharp excitonic intermediate states on depolarized and polarized scattering gives direct evidence for the collective behavior of the spin-density excitation in contrast to the band of single-particle excitations. The observed spin-density energy (0.9 meV) is at significantly lower energy than the single-particle energy (1.1 meV). This is due to the Coulomb exchange interaction in the electron gas. © 1990 The American Physical Society.