Abstract 1. 1. Serum proteins from sarcornatous soft shell clams, Mya arenaria L., enhanced transmission of sarcoma. 2. 2. Sarcoma cells were isolated and administered to the recipients at the same cell density in different sarcoma-protein-free diluents: seawater, serum from normal clams, heat-treated sarcoma serum or protease-digested sarcoma serum. 3. 3. Transmission in these groups was significantly slower than in the group where cells were administered in intact sarcoma serum, demonstrating that the tumor promoting factors in the serum were heat-sensitive proteins. 4. 4. Normal hemocytes administered in sarcoma serum caused mortality but not sarcoma transmission, suggesting the presence of cytotoxic factors in sarcoma serum.