Immune spleen cells or activated thymus cells were incubated with appropriate51Cr‐labelled target fibroblasts. The interaction of lymphoid cells and target cells was stopped by fixation in formalin vapour. Target cell lysis was evaluated by the release of51Cr and the cultures were stained for acid phosphatase activity by the simultaneous capture technique, using naph‐thol AS‐BI phosphoric acid and Fast red‐violet LB salt. The sensitized lymphoid cells aggregated around cultured donor‐type fibroblasts within a 4‐hour incubation period and staining for acid phosphatase activity appeared in the lymphocytes. The reaction was immuno‐logically specific, as staining was observed only in the immune spleen cells or activated thy‐mocytes incubated with target cells possessing the allo‐antigens to which the cell donor was sensitized. Specific adsorption on fibroblast monolayers syngeneic, but not allogeneic, to the H‐2 allo‐antigens against which the thymus cells were activated, reduced both the specific release of51Cr from labelled target cells and the number of thymus cells staining for acid phosphatase activity. Acid phosphatase staining and cytolytic activity were reduced in lymphocyte‐target cell cultures incubated in the presence of prednisolone or heparin. A significant correlation was found between target cell lysis and number of acid phosphatase‐positive spleen cells in relation to time of interaction and lymp