The suppression of Zeeman energy splitting due to spin-dependent interactions (the spin Meissner effect) was predicted to occur within a Bose-Einstein condensate.We report a clear observation of this effect in semimagnetic microcavities which exhibit a giant Zeeman energy splitting between two spin-polarized polariton states as high as 2 meV and demonstrate that a partial suppression of the energy difference occurs already in the uncondensed
phase in a striking similarity to the behavior of up-critical superconductors in the fluctuation-dominated regime. These observations are explained quantitatively by a kinetic model accounting for both the condensed and uncondensed polaritons and taking into account the nonequilibrium character of the system.