The characteristics of the nuclear ribosomal protein prepared by acid-extraction of calf thymus nuclear ribosomes have been studied. The protein shows similar amino acid composition as the total protein of the whole nuclear ribosomes and is therefore basic. Sedimentation analysis of the protein shows mainly species of 0.9 S. An average molecular weight of 20 800 was obtained by the Archibald method. However, starch-gel electrophoresis experiments indicate that the protein is highly heterogeneous, containing at least 13 components. By comparison, cytoplasmic ribosomal protein prepared from calf thymus shows relatively fewer components than the nuclear preparation. Immunochemical interactions between the acid-soluble nuclear and cytoplasmic ribosomal proteins with antisera prepared against the nuclear and cytoplasmic ribosomes show that while the nuclear ribosomal protein cross-reacts with cytoplasmic antiserum equally well as with nuclear antiserum, the cytoplasmic ribosomal protein is less active when cross-reacted with nuclear antiserum. The results suggest that the nuclear ribosomes differ from their cytoplasmic counterpart by containing more protein components.