The present invention discloses modified Staphylococcus aureus HIa proteins which show reduced tendency to aggregate, improving protein stability and yield. Said modified HIa proteins optionally also contain glycosylation site consensus sequences. The invention also discloses a conjugate comprising a modified HIa protein and an antigen (for example a Staphylococcus aureus saccharide antigen), wherein the antigen is linked to an amino acid residue of the modified HIa protein.