A study was made on alterations in the surface structures of Gross virus-induced murine thymic leukemia cells and budding virions with freeze-etching technique. The budding virions were clearly observed on the fractured faces of cell surface as well as microvilli and ruffled membranes. Knobs, approximately 10 nm in diameter, were demonstrated on the surfaces of budding virions. Extracellular virions had also the similar structure on their surfaces. On the fractured protoplasmic face (P.F.), 8.5-10 nm typical intramembranous particles (I.P.) were arranged randomly without detectable aggregated particles. There also appeared smooth semispherical protrusions which could be regarded as budding virions. Comparison was carried out between numbers of I.P. in leukemia cells and those in normal thymocytes, showing that I.P. slightly increased in leukemia cells.