SUMMARY Characterization of the serum components which support lymphocyte-mediated cytolysis in vitro has revealed that they are heat-stable and associated with high molecular weight species. The presence of normal serum increases the rate of cytolysis, which can, however, occur to a lesser extent in serum-free media. Serum which can facilitate cytolysis in the presence of glucose has been found to be inhibitory in its absence. This inhibitory effect is attributable to an irreversible interference with the cytolytic function of the effector cells. Therefore, serum should be considered as one of a number of factors which can influence the extent of lymphocyte-mediated cytolysis.