Chemical analysis of Semliki Forest virus grown in BHK21 cells revealed 6.3 % RNA, 12.2 % nucleocapsid protein, 44.4 % envelope polypeptides, 30.8 % lipid and 6.3 % proteinbound carbohydrate. The monosaccharides found in the virus include N-acetyl-glucosamine, N-acetyl-galactosamine, mannose, galactose, glucose, fucose and sialic acid. Analyses of the individual monosaccharides are presented. The number of molecules of each of the major constituents in an average virion was calculated.