The nucleotide and amino acid compositions of ribosomes prepared from isolated calf thymus nuclei have been determined. Nuclear ribosomal ribonucleic acid (RNA) from calf thymus is characterized by a high content of cytidylic and guanylic acids, as is cytoplasmic ribosomal RNA of animal origin. However, both nuclear and cytoplasmic ribosomes of calf thymus cells show a higher cytidylic acid to guanylic acid ratio than ribosomes of other types of cells. The amino acid composition of thymus nuclear ribosomes is very similar to that of cytoplasmic ribosomal proteins obtained from other cell species. Nuclear ribosomes, cytoplasmic ribosomes, and certain nuclear proteins are compared with respect to amino acid composition.