1. Chicken brain arylsulphatase A was purified 2000-fold, with overall recovery 14%, by using ammonium sulphate fractionation, ethanol precipitation, Sephadex G-200 gel filtration and DEAE-Sephadex column chromatography. 2. The purified preparation was free from β-glucuronidase, β-galactosidase, acid phosphatase, inorganic pyrophosphatase and adenosine 3′-phosphate 5′-sulphatophosphate sulphohydrolase activities. 3. Polyacrylamide-gel electrophoresis indicated that the purified preparation was not homogeneous. 4. Chicken brain arylsulphatase was markedly inhibited by carbonyl reagents in the presence of traces of Cu2+ in the system. Other metal ions such as Fe2+ and Zn2+, were inactive. 5. Ascorbic acid alone had no effect on enzyme activity but enhances the inhibition by Cu2+. 6. Chicken brain arylsulphatase A resembled arylsulphatase A of other animal species in its kinetic properties such as Km value, anomalous time–activity relationship and the inhibitory effect of phosphate, sulphite and sulphate ions. However, its electrophoretic mobility, behaviour under zinc acetate fractionation and stimulation by Ag+ were similar to arylsulphatase B of other animal species. Thus, this enzyme did not correspond to either arylsulphatase A or arylsulphatase B but properties of both. 7. The purified enzyme preparation can degrade cerebroside 3-sulphate.