Photooxidation of hydroxylamine to nitrite by spinach (Spinacia oleracea L. and sugarbeet (Beta vulgaris L.) chloroplast lamellae in the presence of autoxidable electron accepters is inhibited by either solubilized or membrane-bound superoxide dismutase (SOD). This inhibition is reversed by KCN. The rates of hydroxylamine photooxidation by chloroplast lamellae, a reaction which is apparently driven by the superoxide free-radical ion, was used for quantitating the amount of SOD bound to chloroplast lamellae, as compared to a soluble enzyme of defined concentration. After digitonin fragmentation of chloroplast lamellae, ca. 80% of the SOD activity is associated with subchloroplast particles sedimenting after 2 h centrifugation at 200 000 )<g. Less than 10% of the SOD activity is associated with particles sedimenting after centrifugation for 30 rain at 20000 x g. 5-10% of the cyanide-sensitive SOD is recovered in the soluble fraction of the subchloroplast-free supernatant after centrifugation at 200000 X g for 2 h.