Sulphatase A was first described as an arylsulphatase but was subsequently shown to have cerebroside sulphatase activity, bydrolysing galactose 3-sulphate residues in certain lipids. A characteristic feature of the arylsulphatase activity is the substrate-induced inactivation of the enzyme which occurs during the catalytic reaction. Present views of the course of this modification are considered. A similar modification of the enzyme does not occur during the cerebroside sulphatase reaction and the fall in velocity noted during most such assays can be explained by disappearance of substrate and accumulation of sulphate. Possible reasons for the difference between the two types of activity are considered. Sulphatase A also hydrolyses hexose sulphates at rates comparable to those of aryl sulphates. The specificity of sulphatase A towards its natural substrates, sulpholipids, is considered in the light of these findings.