Acid phosphatase was purified to electrophoretic homogeneity from human normal lung and spleen and was characterized biochemically and immunologically in comparison with prostate acid phosphatase (PAP). The apparent MW of lung acid phosphatase (LAP) and spleen acid phosphatase (SAP) was 11 0,000 and 100,000, respectively, similar to that of PAP (100,000). All three enzymes exhibited similar electrophoretic mobility, optimal pH, substrate, and inhibitor specificity, except that PAP dephosphorylated profoundly the phosphate group from tyrosine phosphate in phosphoangiotensin (1 9,700 fmoVmg/min), whereas only marginal activities were detected for LAP and SAP (19 and 73 fmol/mg/ min, respectively). Amino acid analysis revealed more similarity between SAP and LAP than PAP and LAP or PAP and SAP. An immunological cross-reactivity among these three acid phosphatases was detected by polyclonal and monoclonal antibodies raised against purified PAP, although unique epitopes were detected on the PAP molecule. This study provides data explaining why conventional biochemical methods are not specific for PAP measurement and why immunologic methods still detect other acid phosphatases, as observed in clinical laboratory assays. The data also suggest the possibility of using a new substrate or antibody reagent for a more specific assay for PAP.