A method to prepare photosystem I (PSI) particles is described. Spinach leaves are used to prepare broken chloroplasts that are then solubilized by using a detergent (Triton X-100). Solubilized chloroplasts are then applied on an ion-exchange column. Eluted by a linear concentration gradient of NaCl, fractions enriched in PSI particles are collected and applied on a small hydroxyapatite column. By eluting with phosphate buffer, a concentrated preparation of PSI particles is obtained. The particles consist of PsaA, PsaB, PsaC, PsaD, PsaE, and PsaY. Assay methods that involve SDS-PAGE and P700 determination are also presented.