Abstract The ribosomal proteins of the yeast Saccharomyces cerevisiae have been examined by gel electrophoresis. The large subunit has about 50 proteins, with molecular weights ranging from 10,000 to 46,000; the small subunit has about 30 proteins, with none larger than 30,000. Newly formed ribosomal protein appears on ribosomes during a period of about 20 min after its synthesis. At this time much of the ribosomal protein is within the nucleus, where it may be associated with maturing RNA in the form of partially assembled ribosomes. For a short time after a pulse of radioactive amino acid, one can observe on ribosomes a distinct set of proteins which are probably not involved in the assembly of new ribosomes. Newly formed ribosomes first appear in the cytoplasm as subunits. Although the new subunits are indistinguishable from the old ones on the basis of their buoyant density and the acrylamide gel analysis of their proteins, they pass rather slowly into the polysomes, which suggests that a final cytoplasmic maturation step is required. The process of ribosome synthesis in yeast is nearly indistinguishable from that in higher organisms. This finding validates the attempt to use a simple, genetically tractable organism, the yeast, to study the general problem of nucleocytoplasmic interactions in eukaryotes.