Spin-polarized first-principles total energy calculations have been developed to explore the Mn and Ge monolayer adsorption and incorporation into the unreconstructed GaAs(111)-(1 x 1) A surface. Surface formation energy results show that a germanium terminated surface is stable in a wide range of chemical potential; in this structure, the first Ga layer is replaced by a Ge layer. Upon increasing the As coverage, it is possible to find a stable structure with a manganese bilayer under an As monolayer. The density of states shows that a Ge terminated structure displays a semiconductor behavior whereas the Mn bilayer is metallic, with evident antiferromagnetic characteristics induced by the Mn atoms. Mn magnetic moments are of the order of similar to 4 mu(B). The magnetic arrangement was confirmed by spin density distribution. Such antiferromagnetic structure may be suggested as a substrate to form magnetic junctions with other magnetic materials.