1. 1.|The rate of release of high molecular weight water-soluble components from isolated chloroplasts has been studied. 2. 2.|Chloroplasts have been broken by osmosis and successively washed with hypotonic buffer solutions, or ruptured in a needle-valve disintegrator to free lamellae particles of soluble constituents. The resulting solubilized material has been examined by ultracentrifugation, and by polyacrylamide gel electrophoresis in homogeneous buffer system. 3. 3.|Fraction I protein was more readily liberated from the chloroplasts than the Fraction II components. A component of s 20, w = 12 S was observed in some of the extracts, but the bulk of this was only liberated after complete disruption of the lamellae, and it is thought to be situated within the lamellae ‘loculi’ and ‘fret-channels’. 4. 4.|A procedure for the purification of Fraction I protein using DEAE-cellulose chromatography and gel filtration is described. 5. 5.|An investigation of the nature of our preparation of Fraction I protein is reported; the protein has a partial specific volume of 0.744 ml/g, and an s° 20, w = 18.30 S. A minimum molecular weight of 24427 has been calculated from the amino acid analysis, and the molecular weight of the macromolecule has been determined by ultracentrifugation (on two preparations) as 585 000 and 561 000. The protein was observed to dissociate into subunits of 2.6–4.8 S, in alkali (pH 11.5), acetic acid (70%), urea (8 M) and sodium dodecyl benzene sulphate (detergent-protein, 1:2, w/w). 6. 6.|The isolated Fraction I protein contains 84% protein; the other material present is thought to be accounted for by carbohydrate and material giving rise to ash. The predominant monosaccharides in the hydrolysates of the protein have been identified as glucose and xylose. 7. 7.|Fraction I protein has been shown to be identical with the enzyme, ribulose-1,5-diphosphate carboxylase (EC 4.1.1.39). Correspondence between the physical properties of Fraction I protein and those of protochlorophyll-protein complex of etiolated tissues indicates another possible identity.