Material with a molecular weight of less than 10(5) daltons has been isolated and partially purified from the ascitic fluid of DBA2 mice bearing a syngeneic lymphoma (SL2). This substance inhibits the cytotoxic action of an allogeneic antiserum directed specifically against SL2 cells. Material has been rendered radioactive with 125I and between 20 and 25% of the radioactivity is bound in a specific manner to the antiserum. The material which is referred to as 125I-TSTA has been used in a radioimmunoassay to measure the level of TSTA in the sera of mice bearing both ascitic and subcutaneous SL2 tumours. The level of circulating TSTA was found to be high immediatley following inoculation of live SL2 cells, probably because a large proportion of the injected cells autolyse. The serum concentration of TSTA then falls but 6-10 days later begins to rise again in parallel with the growth of the SL2 tumour either in the peritoneal cavity or subcutaneously. Following surgical removal of an intradermal SL2 tumour the level of TSTA in the serum falls rapidly. No evidence could be found that a significant proportion of the TSTA in the serum of tumour-bearing mice is completed with antibody. However, in the serum of DBA2 mice which have been hyperimmunized with irradiated SL2 cells there are antibodies which bind 125I-TSTA although syngeneic anti-SL2 sera, unlike alloantisera, do not show complement dependent lysis of SL2 cells
