The invention provides a lipid of general formula (I) or (II): ##STR00001## wherein X.sup.1, X.sup.2 and R.sup.1 to R.sup.5 are as defined herein. Such lipids are used to form complexes with a biologically-active material such as a nucleic acid, peptide or small molecule for delivering the biologically-active material to cells. The complexes may incorporate an integrin-binding peptide and, when the biologically-active material is DNA, thereby constitute a LID complex.