Abstract: A radiolabeled lymphatic staining agent is mixed on site with a much larger amount, on a weight basis, of a non-radiolabeled lymphatic staining agent to form an injectable radiolabeled lymphatic staining agent suitable for surgical use in humans. Preferably, the radiolabel is I-125, because it has a 60-day half-life which enables it to be made off-site. The preferred radiolabeled lymphatic staining agent is iodinated methylene blue, because it can be mixed with a range of non-radioactive lymphatic staining agents, for example isosulfan blue, methylene blue, patent blue, and patent blue V, to provide the injected agent with sufficient radioactivity to enable machine detection. A method for making radioiodinated methylene blue is also disclosed.