The present invention relates to insulin derivatives which are naturally occurring insulins or analogues thereof which have a side chain attached either to the α-amino group of the N-terminal amino acid residue of the B chain or to the ε-amino group of a Lys residue present in the B chain of the parent insulin, the side chain being of the general formula: —W—X—Y—Z wherein W, X, Y and Z are as defined in the disclosure.