One hundred and five scintiscans of the mammary lymphatic system were performed in 100 patients with breast cancer by intratumoral injection of 99 m Tc-labelled colloidal rhenium. The progression of the radioactive colloidal agent was followed on a series of films taken 1, 2 and 4 hours after the injection, and the images obtained were compared with post-operative findings of lymph node involvement. Patients with more than 3 carcinomatous lymph nodes had less than 2 foci of activity, while those with less than 3 carcinomatous lymph nodes had more than 2 foci of activity, owing to more rapid progression of the compound. The difference was highly significant (p less than 0,001).