A planar semiconductor microcavity in the strong coupling regime has been s
tudied by spectrally resolved degenerate four wave mixing using femtosecond
laser pulses. At high excitation densities, new spectral features observed
besides the cavity polaritons can be uniquely identified as of biexcitonic
origin. We investigate the dispersion of these features with cavity detuni
ng. The observations are compared to several model descriptions for the int
eraction of the biexciton with the cavity light field.