First-principle electronic structure calculations are used to describe the magneto-optical properties of MnBiAl alloys as a function of the Al concentration. The calculated Kerr rotation and ellipticity are in good agreement with the experimental data for MnBi. Due to lack of good structural information, the magneto-optical properties of MnBiAl are studied for Al as (a) an interstitial and (b) partially substituting for Bi. The Kerr rotation of MnBi degrades with interstitial Al but remains about the same for a small amount of substitutional Al. The latter result is in agreement with the experimental data. Thus, both theory and experiment are in agreement that Al does not improve the magneto-optical properties of MnBi. \textcopyright{} 1996 The American Physical Society.