N-methyl-N'-methyl-O-(m-PEG4)-O'-(acid-PEG5)-Cy5 is a PEG derivative containing a cyanine dye with excitation/emmission maximum 649/667 nm and a free terminal carboxyl acid. Terminal carboxylic acid can react with primary amine groups in the presence of of activators (e.g. EDC, or DCC) to form a stable amide bond. The hydophilic PEG spacer increases solubility in aqueous media.