A one-step procedure for sentinel lymph node identification and biopsy using a single compound, a radiolabeled, low molecular weight dye (e.g., .sup.125I-labeled methylene blue). This radiolabled dye is mixed with an unlabeled, similar molecular weight dye (e.g., isosulfan or methylene blue). The mixture is injected at the time of surgery, and rapidly migrates to reach the lymph nodes in less than 20 min, more preferably in less than 15 min and most preferably in less than 10 min. Using rabbits, rapid transit of .sup.125I-methylene blue to regional lymph nodes with limited systemic biodistribution has been confirmed. By admixing small amounts of radiolabeled dye with a large amount of unlabeled dye, the sentinel lymph node identification was similar to that for the prior two-step dual mapping process, but with enhanced SLN localization because of the lower energy gamma emission of .sup.125I as compared with .sup.99mTc.
