A method is described for the growth of Semliki Forest virus in suspensions of primary chick embryo fibroblasts and for its subsequent purification. The overall purification of the virus was approximately 250-fold with a 40 to 50% recovery of haemagglutination activity. Analysis of purified Semliki Forest virus by SDS-polyacrylamide gel electrophoresis showed the presence of two distinct protein bands. The protein of the virus envelope has a molecular weight of approximately 51,000 and is a glycoprotein. The lighter protein, with a molecular weight of approximately 32,000, is associated with the nucleocapsid core of the virion and does not appear to contain carbohydrate. Preparative SDS-polyacrylamide gel electrophoresis through a discontinuous gel system yielded mg. quantities of each of the two structural proteins. Amino acid analysis revealed that the envelope protein is relatively rich in hydrophobic amino acids, whereas the core protein is rich in hydrophilic amino acids, particularly lysine and glutamate. The N-terminal amino acid of the envelope protein is valine and that of the core protein is lysine. By employing the dansyl technique, tryptic peptide maps of the envelope and core proteins of Semliki Forest virus were obtained. Approximately 25 soluble tryptic peptides were obtained from the envelope protein and approximately 38 from the core protein.