The physical and chemical characteristics of the proteins which compose virus capsids are important both for consideration of the unique structural properties of the virus coat and to enable an estimate to be made of the size of that part of the genome which specifies other functions. The subunits of the capsids of plant viruses are small proteins of molecular weight 17,000 to 60,000 and the evidence to date indicates that for each plant virus one protein consisting of a single polypeptide chain serves to form the capsid(1). In contrast four distinct protein components, or four polypeptide chains, have been found in poliovirus (2) and five in adenovirus protein (3). The present report describes a preliminary examination of the number of com- ponents in the capsid protein of the DNA-containing animal virus of polyoma.