1. An improved method of measurement of the protein disulphide isomerase (protein disulphide-isomerase, EC 5.3.4.1) activity of microsomal preparations is described. 2. This enzyme is shown to be released from the membranes into solution in the ultracentrifuge. 3. Some of the properties of the enzyme in the membrane bound and soluble forms are described. 4. The results lead to the suggestion that the enzyme present in rough membrane is different from the present in smooth membrane as well as being differently situated in the membrane. 5. Some of the effects of sucrose on the activity of the membrane bound enzyme are consistent with the view that the membranes may undergo irreversible conformational changes during preparation and storage.