pestle, with 0.15 M sodium phosphate buffer, pH 7.5, added in 2-ml portions to a final volume of 2 ml of buffer per g of nodules. The homogenate was filtered through two Miracloth disks in a 50-ml syringe into a 30-ml centrifuge tube. The tube was capped tightly and centrifuged at 6,000 x g for 10 min. The tube was opened in the glove box, the supernatant was discarded, and the pellet was washed twice by pipetting 1 ml of the buffer over it. The pellet was resuspended, and the mixture was again filtered through Miracloth disks to remove any unsuspended bacteroids and adjusted to a final concentration of 2 ml of buffer per g of nodules. This (16), and 0.3 ml of 0.15 M sodium phosphate buffer, pH 7.5. The tubes were flushed with N2 and sealed with a serum stopper with an attached cup containing a filter paper wick impregnated with 70 p.l of 10% KOH. A sample (1.4 ml) was removed from the gas phase and replaced with 1.4 ml of , giving a final concentration of 2% 02 in the gas phase. Buffered substrate (0.5 ml; 1 ,umol at 1 ,uCi/,umol) was then injected into the reaction mixture, and the tubes were shaken in a rotary shaker at 250 rpm. Incubation was terminated by placing the tubes in an ice bath. The stoppers were quickly removed, and the tubes were centrifuged at 25,000 x g for 10 min. The supernatant was discarded, and the pellet was washed twice by pipetting 1 ml of the cold buffer over it and discarding the washes. The