Sulphite is oxidised in the presence of AMP by an enzyme present in the soluble fraction of the cell. In the presence of ferricyanide, sulphite and AMP readily formed APS in the crude extract. However, when ferricyanide was replaced by nitrate and FMN under anaerobic conditions the yield of APS was only about 10% of that obtained with ferricyanide. APS reductase was purified 35-fold. The purified enzyme was inhibited by thiol-binding reagents. Its molecular weight was estimated to be 250,000.