LaCrosse virus, a member of the California encephalitis complex, was grown in BHK-21, clone 13 cells and purified by precipitation with 20 M polyethylene glycol and isopycnic banding at 1.18 g/cc in linear 15–60% sucrose gradients. Three polypeptides with molecular weights of approximately 26,000, 45,000, and 85,000 daltons were distinguishable in the virus upon SDS-polyacrylamide disc electrophoresis. Radioactive labeling in a reduced amino acid medium, however, caused the reduction or loss of the 45,000 dalton polypeptide. The LaCrosse virus contains single-stranded RNA which is infectious. The viral RNA was resolved into six peaks with sedimentation coefficients of approximately 4 S, 16 S, 20 S, 27 S, 35 S, and 50–60 S. Heating the RNA at 80 °C for 2.5 min dissociated the larger RNA classes (35 S and 50–60 S) and produced a broad 20 S peak ranging from 4 S to 27 S.