Analysis of membrane carbohydrates has revealed that following SV-40 virus transformation of mouse fibroblast 3T3, there is a marked decrease in most neutral and amino sugars, especially sialic acid and N-acetylgalactosamine. To examine this change in the composition of membrane glycoproteins in greater detail, we have fractionated the labeled membrane fractions derived from 3T3 and transformed 3T3 lines on Sephadex G-150 in the presence of sodium dodecyl sulfate. In addition, membrane glycoproteins were digested with the proteolytic enzyme, Pronase, followed by the fractionation of the glycopeptide mixture on Sephadex G-50. The possible mechanisms by which these changes in the composition of membrane glycoproteins are brought about are discussed. © 1969, American Chemical Society. All rights reserved.