1. In vitro incubation of Potter-Pilgrim No. 5563 and No. 5647 mouse myeloma tumors with tritiated-d,l-leucine produced demonstrable radioactivity in the respective myeloma proteins.
2. In vivo inoculation of H3-d,l-leucine in tumor bearing animals showed similar incorporation and in addition showed incorporation of radioactivity into albumin and alpha globulin.
3. It is suggested that the in vitro studies better serve to identify the isolated tissue function in protein synthesis.
4. Mouse myeloma studies confirm previous results in human multiple myeloma and indicate that in vitro incubation of protein producing tissues with H3-d,l-leucine can demonstrate protein synthesis.