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Iron was resupplied to tobacco that was severely iron-deficient and the chloroplasts investigated for chlorophyll, iron content, photochemical, and development aberrations and photosystem components. Ferredoxin was purified from normal and iron deficient tobacco leaves. Lower ferredoxin and chlorophyll content in iron-deficient leaves were found compared to normal leaves. The ferredoxin from normal and iron-deficient leaves had the same molecular weight as spinach ferredoxin (Sigma Chemical Co.) and shared a similar antigenic property.Photosystem I (PSI) acti vity increased by about 100% during the first 10 days of regreening on a leaf surface area comparison. Only small increases in activity were found with PSII. Purified PSI particles were spectrophotometrically examined for any changes in P700, chlorophyll, cytochrome and protein components on a leaf area basis. With greening the individual components increased but the ratios remained constant. There were 6 polypeptide bands in the PSI particles when examined by electrophoresis. All bands stained with equal intensity with coomassie blue except for the ISkD band at day 0 of greening. Data suggest that iron stress is involved in the regulation of PSI development, possibly by the direct regulation of a low-molecular weight protein required for system assembly. Discussion is presented suggesting this protein is ferredoxin and that limitation of this component would directly affect chlorophyll biosynthesis and chloroplast structure.