The purpose of these investigations was to isolate and purify the largest of several antigens demonstrable in rabies-infected suckling mouse brains. The biochemical and biophysical properties of the antigen were studied with a view to elucidating its contribution to the intracellular synthesis and the structure of the virus particles. Extracts of normal and infected suckling mouse brains were purified by precipitation at pH 4.5 and freed of the smaller antigens by centrifugation prior to digestion with RNAase, DNAase and trypsin. The large antigen was purified by enzyme treatment, preparative ultracentrifugation, exclusion chroma tography and gradient centrifugation and appeared as rings varying in diameter between 8 and 12 mμ when examined by electron microscopy. Methods for the chemical estimation of pentose, deoxypentose and nitrogen were modified to meet the requirements of this investigation, and these techniques were used to determine the composition of the purified antigen. The antigen is a ribonucleoprotein, and found to be resistant to RNAase, DNAase, trypsin and chymotrypsin. A purified solution of the antigen contained 7.3μg RNA/ml., 11.4μg protein/ml and probably a trace of DNA. The success of this programme has resulted in the accumulation of certain original information which has been used in clari£ying the nature and structure of the largest soluble antigen.