We used isotope dilution MS to measure the stoichiometry of light-harvesting complex I (LHCI) proteins with the photosystem I (PSI) core complex in the green alga Chlamydomonas reinhardtii. Proteotypic peptides served as quantitative markers for each of the nine gene products (Lhca1-9) and for PSI subunits. The quantitative data revealed that the LHCI antenna of C. reinhardtii contains about 7.5 +/- 1.4 subunits. It further demonstrated that the thylakoid LHCI population is heterogeneously composed and that several lhca gene products are not present in 1:1 stoichiometries with PSI. When compared with vascular plants, LHCI of C. reinhardtii possesses a lower proportion of proteins potentially contributing to far-red fluorescence emission. In general, the strategy presented is universally applicable for exploring subunit stoichiometries within the C. reinhardtii proteome.