Mechanisms for localization of radionuclides in soft tissue tumors are discussed in relationship to a sequential observation of the label from the time it enters the intravascular space until the time it can be identified in a tumor in greater concentration than in surrounding normal structures. In the case of soft tissue lymphoma identification with gallium or other labeled compounds, it appears that the degree of positivity on a scan will depend upon the characteristic features that the tumor has which differ from the normal surrounding structure. These features include the relative blood volume in the tumor, the transcapillary transport of labeled protein within the tumor, the relative pinocytotic activity of tumor cells in relation to normal tissue, the relative affinity for the label of intracellular proteins in tumor cells vs. normal tissue, and finally the capacity of the tumor to retain the radioactive label due to poor lymphatic drainage or interstitial pressure different from surrounding tissue. By appreciating these possible mechanisms, we may now be able to search for those features which would lead toward positive tumor scans and predict the characteristic features of tumors which will make it possible to detect them by tracer methods. In recognizing these features, we may be able to develop tracer techniques to improve the identification of soft tissue neoplasms.