Sensitivity of mammalian cells to the bacterial toxin aerolysin is due to the presence at their surface of glycosylphosphatidyl inositol (GPI)-anchored proteins which act as receptors. Using a panel of mutants that are affected in the GPI biosynthetic pathway and Trypanosoma brucei variant surface glycoproteins, we show that addition of an ethanolamine phosphate residue on the first mannose of the glycan core does not affect binding. In contrast, the addition of a side chain of up to four galactose residues at position 3 of this same mannose leads to an increase in binding. However, protein free GPIs, which accumulate in mutant cells deficient in the transamidase that transfers the protein to the pre-formed GPI-anchor, were unable to bind the toxin indicating a requirement for the polypeptide moiety, the nature and size of which seem of little importance although two exceptions have been identified.