Technetium 99m is readily available from a99Mo-99mTc generator and possesses nearly ideal physical properties for organ visualization when used with either a scanner or scintillation camera. Brain phantom studies indicate that comparable resolution of pseudotumors containing99mTc may be obtained with either unit. Studies of more than 100 patients suspected of having brain lesions indicated that99mTc in the pertechnetate form is satisfactory for cranial scintiphotography. Albumin is tagged radioactively with99mTc in our laboratories and used for placental localization. A colloid of this material is obtained by controlling heating. Small particles (200μ) are used for visualization of liver and spleen. Macroaggregates (50μ to 100μ) obtained by continued heating are used for lung scintiphotograms. Animal and human studies with preparations of albumin labeled with radioactive technetium (99mTc) have yielded good results with no adverse side effects noted.