NIH3T3 cells were transfected with a vector containing c-H-ras under the transcriptional control of the mouse metallothionein-I promoter. When c-H-ras expression was induced with heavy metal ions there was a marked reduction in the expression of two glycosylphosphatidylinositol (GPI) anchored proteins, TAP/Ly-6A.2 and Thy-1, on the plasma membrane of the transformed cells. In contrast the cell-surface expression of other non-GPI-anchored proteins was unaltered. The major loss of Thy-1 induced by ras activation occurred from the pool of molecules expressed on the cell surface. The Thy-1 molecules that were preferentially lost from the surface of the ras-transformed cells could not be recovered from the extracellular fluid by immunoprecipitation. In contrast, the rate of internalization of Thy-1 was increased approximately 69% subsequent to ras activation. The possible significance of these findings to the function of c-H-ras is discussed.