SUMMARY Interactions of humoral antibodies (C57 anti-BP8) and syngeneic immune lymphocytes have been studied in vivo. The effect of each of these agents alone, or in combination, on the growth of the ascites sarcoma BP8 (C8H tumor) in C57 mice was determined. Small amounts of antiserum alone regularly produced enhancement of tumor growth, whereas C57 lymphocytes obtained early after immunization, or large volumes of antiserum alone, were inhibitory to tumor growth. Sensitized C57 lymphocytes obtained at a later time after immunization frequently produced tumor enhancement under the same experimental conditions. Synergistic action was demonstrated between immune serum and such older antibody producing lymphocytes, whereas antagonism was noted between immune serum and more recently sensitized lymphocytes. The importance of these observations in relation to the mechanism of allograft rejection is discussed.