We describe a near-infrared fluorescent heptamethine dye (IR-780 iodide) with unique properties for sentinel lymph node (SLN) mapping in both small and large animals. This dye has a significant photobrightening effect in serum and a long retention time in the lymphatic system which allows to acquire much higher signal-to-noise ratios. Injection of only 10 nmol of this dye permits SLNs to be imaged easily in pigs using excitation fluence rates of only 2 microW/cm(2). In addition, this dye has a unique stability property after formalin fixation in tissues which raises the possibility of developing new and sensitive means of detecting lymph nodes in harvested surgical specimens. This dye can be completely cleared from the circulation in a couple of days and does not cause acute systemic toxicity.