Abstract The membrane glycoproteins E 1 and E 2 of Semliki Forest virus form spikes protruding from the external surface of the virion. They have been cleaved off by thermolysin or subtilisin leaving peptide segments in the membrane of the spikeless virus particles with a molecular weight of about 5000 enriched in hydrophobic amino acids. These peptides are soluble in chloroform/methanol and are solubilized into mixed micelles with Triton X100, with sodium dodecyl sulphate and with sodium deoxycholate. Peptide mapping studies show that each membrane glycoprotein has its own lipophilic peptide segment which presumably serves to anchor these proteins to the lipid membrane. The hydrophobic segments of the glycoproteins appear to be shielded from proteolysis not only by the lipids in the intact membrane but also by Triton X100 in the detergent-protein complexes obtained when this detergent is used to remove the lipid and solubilize the proteins.