In the presence of specific antigen, lymph node cells from inbred rats with delayed hypersensitivity to tuberculoprotein, bovine gammaglobulin, and egg albumin produced progressive destruction of monolayers of rat embryo fibroblasts in tissue culture, first apparent at 48 hr and maximal at 72 hr. The effect was specific and did not depend on a genetic difference between the lymph node cells and target cells. It required antigen concentrations equal to or greater than 1.25 µg/ml and lymphocyte: target cell ratios of approximately 10 or 20:1. It could be evaluated both by a plaquing technique and by cell enumeration with an electronic particle counter.

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