To identify new proteins involved in Mn2+ homeostasis, we isolated Mn2+-resistant mutants of Saccharomyces cerevisiae starting from a calcineurin-deficient, Mn2+ hypersensitive strain (Delta cmp1 Delta cmp2). The mutations were found to lie in the PMR1 gene, known to encode a "P-type" Ca2+-ATPase that transports Ca2+ and Mn2+ from the cytosol to the Golgi apparatus. A second gene, AHP1, was cloned as a suppressor of the Mn2+ tolerance of a Delta cmp1 Delta cmp2 pmr1 mutant. Ahp1p was recently described as a thioredoxin peroxidase type II, an antioxidant protein with alkyl hydroperoxide defense properties in yeast. AHP1 disruption in strain W303 decreased tolerance to Mn2+ and H2O2. We found that a GFP-Ahp1p fusion construct was in the cytosol when cells were grown in glucose, and in the mitochondria when cells were grown in oleate. Based on Mn2+ transport data, we concluded that Ahp1p is involved in cellular Mn2+ homeostasis in trafficking of Mn2+ from cytosol to mitochondria and from cytosol for export across the plasma membrane.