1. N-Acetylgalactosamine 6-sulphate sulphatase was purified about 20000-fold from the soluble extract of human placenta with N-acetylgalactosamine 6-sulphate-glucuronic acid-N-acetyl[1-(3)H]galactosaminitol 6-sulphate as substrate in the activity assay. The enzyme appears to be a glycoprotein with a mol.wt. of about 100000 as determined by gel filtration. On gel electrophoresis in the presence of sodium dodecyl sulphate the major protein band had a mol.wt. of 78000. Variable charge heterogeneity was observed in several enzyme preparations. 2. The purified enzyme released up to one sulphate molecule from the disulphated trisaccharide. It was active towards N-acetylgalactosamine 6-sulphate and exhibited no measurable N-acetylglucosamine 6-sulphate sulphatase or any other known lysosomal sulphatase activity. Hydrolysis of [1-(3)H]galactitol 6-sulphate was achieved by incubation neither with a crude nor with a purified enzyme preparation. Chondroitin 6-sulphate and keratan sulphate, as well as heparin and heparan sulphate, served as competitive inhibitors of the enzyme. 3. Purified N-acetylgalactosamine 6-sulphate sulphatase activity was optimal at pH4.9 and 4.4 when assayed in 0.02m-sodium acetate buffer and at pH4.2 and 5.2 in 0.1m-sodium acetate buffer. A single pH-optimum at pH4.8 was observed for the crude enzyme and for the purified enzyme after mild periodate treatment. The sulphatase activity was inhibited by a variety of anions and cations and activated by thiol-specific and thiol reagents.