The light-harvesting chlorophyll a/b protein (LHCP) is an approximately 25,000-D thylakoid membrane protein. LHCP is synthesized in the cytosol as a precursor and must translocate across the chloroplast envelope before becoming integrally associated with the thylakoid bilayer. Previous studies demonstrated that imported LHCP traverses the chloroplast stroma as a soluble intermediate before thylakoid insertion. Here, examination of this intermediate revealed that it is a stable, discrete approximately 120,000-D species and thus either an LHCP oligomer or a complex with another component. In vitro-synthesized LHCP can be converted to a similar form by incubation with a stromal extract. The stromal component responsible for this conversion is proteinaceous as evidenced by its inactivation by heat, protease, and NEM. Furthermore, the conversion activity coelutes from a gel filtration column with a stromal protein factor(s) previously shown to be necessary for LHCP integration into isolated thylakoids. Conversion of LHCP to the 120-kD form prevents aggregation and maintains its competence for thylakoid insertion. However, conversion to this form is apparently not sufficient for membrane insertion because the isolated 120-kD LHCP still requires stroma to complete the integration process. This suggests a need for at least one more stroma-mediated reaction. Our results explain how a hydrophobic thylakoid protein remains soluble as it traverses the aqueous stroma. Moreover, they describe in part the function of the stromal requirement for insertion into the thylakoid membrane.