The mechanisms of the in vitro interaction of sensitized lymphocytes and allogeneic target cells has been studied in a tumour allograft system in inbred mice. The cytotoxic effect of sensitized lymphocytes is shown to require the presence of Ca(+ +) and Mg(+ +). Pretreatment of the lymphocytes with trypsin led to inhibition of cytotoxicity, followed by spontaneous reversal after 1–3 hours incubation. Reactivation was found to be blocked by an inhibitor of protein synthesis (cycloheximide). Cortisone was not found to inhibit the lytic interaction significantly; an occasional effect is thought to be due to toxicity of cortisone for lymphocytes as revealed by dye exclusion test. Inhibition of DNA-synthesis with FUdR (an inhibitor of the enzyme thymidine synthetase) did not reduce the lytic activity of sensitized lymphocytes. Isologous anti-target cell sera induced in various strains of inbred mice were found to be ineffective in blocking the cellular immune reaction in vitro when directed against a minor part of the antigenic complex, but strongly inhibitory when reactive against a major part or the whole complex. Similarly, target cells lacking several of the sensitizing H-2 antigens were not lysed. An isologous anti-lymphocytic serum induced in the graft donor strain and directed against the recipient strain (lymphocyte donor) did not inhibit the cytotoxic reaction. In a heterologous system on the other hand, the lytic effect of guinea-pig lymphocytes sensitized against mouse target cells was effectively blocked by an anti-lymphocytic serum induced in mice of the graft donor strain by injection of recipient (guinea-pig) spleen cells
