Tartrate-inhibitable acid phosphatase was purified to apparent homogeneity from human placenta. The enzyme is composed of two subunits with an apparent molecular mass of 48 kDa. Each subunit carries one oligosaccharide of the high-mannose/hybride type. The purified enzyme has an isoelectric point of pH 6.2. It cleaves phosphomonoester bonds at acid pH, is competitively inhibited by L-tartrate, Ki = 0.51 microM, and phosphate, Ki = 0.8mM. A monospecific antiserum raised against the purified placental enzyme precipitated 62% and 85% of the tartrate-inhibitable acid phosphatase present in extracts of placenta and fibroblasts, respectively. By means of subcellular fractionation and immunoprecipitation it was shown that the majority of tartrate-inhibitable acid phosphatase is located in lysosomes in normal and mucolipidosis II fibroblasts. In the human Hep G-2 hepatoma cells a significant fraction of the enzyme appears to be associated with non-lysosomal organelles.