This is a new project, just commencing. Extensive studies of Ewing's sarcoma and related round cell tumors of childhood have served to distinguish most from one another on the basis of positive reactivity with one or more antibodies. No antibody specific for Ewing's sarcoma, or restricted to a few tumors including Ewing's, has been identified to date. It is the aim of the present study to produce a battery of monoclonal antibodies against selected, well-characterized Ewing's sarcoma lines established in this laboratory. They will then be studied and compared in their reactivity with other monoclonal antibodies previously reported. Any antibody found to be strongly reactive with Ewing's sarcoma cells will be further characterized, with the eventual intention to use same as a potential diagnostic, imaging, and therapeutic tool.