Sodium dodecyl sulfate gel electrophoresis of unheated, detergent-solubilized thylakoid membranes of Chlamydomonas reinhardtii gives two chlorophyll-protein complexes. Chlorophyll-protein complex I (CP I) is the blue-green in color and can be dissociated by heat into "free" chlorophyll and a constituent polypeptide (polypeptide 2; mol wt 66,000). Similar experiments with spinach and Chinese cabbage show that the higher plant CP I contains an equivalent polypeptide but of slightly lower molecular weight (64,000). Both polypeptide 2 and its counterpart in spinach are soluble in a 2:1 (vol/vol) mixture of chloroform-methanol. Chemical analysis reveals that C. reinhardtii CP I has a chlorophyll a to b weight ratio of about 5 and that it contains approximately 5% of the total chlorophyll and 8-9% of the total protein of the thylakoid membranes. Thus, it can be calculated that each constituent polypeptide chain is associated with eight to nine chlorophyll molecules. Attempts to measure the molecular weight of CP I by calibrated SDS gels were unsuccessul since the complex migrates anomalously in such gels. Two Mendelian mutants of C. reinhardtii, F1 and F14, which lack P700 but have normal photosystem I activity, do not contain CP I or the 66,000-dalton polypeptide in their thylakoid membranes. Our results suggest that CP I is essential for photosystem I reaction center activity and that P700 may be associated with the 66,000-dalton polypeptide.