The specificity of anti-lymphocyte globulin (ALG) has been used to analyze an immune mechanism which is mediated by immunologically committed lymphoid cells to the apparent exclusion of humoral antibody. Rabbit antimouse lymphocyte globulin completely suppressed the immunity which can be passively transferred with Listeria-immune lymphoid cells from actively infected donors. When prospective donors were given a single dose of 1.0 mg of ALG, it remained active against immune lymphoid cells transferred 24 hr later; yet immune cells in the spleens of donors could not be inactivated in situ by even larger doses of ALG given 24 hr prior to cell harvest. In keeping with this finding, the immunity to reinfection with Listeria was not suppressed by a single dose of ALG, indicating that the immunologically active cells in the spleen are not accessible to intravenously administered ALG. On the other hand, protracted treatment with ALG did abolish most of the memory of a previous infection in intact animals. From this and other evidence, it was concluded that immunologically committed cells are vulnerable to attack by ALG only if they circulate. While in circulation, they make contact both with ALG and the phagocytic elements of the reticuloendothelial system which appear to be responsible for their destruction. Four lines of evidence indicated that the suppression of anti-Listeria resistance with ALG depends upon destruction of immune lymphoid cells and not to any action it has on host macrophages. It is possible to infer from this that immunity to L. monocytogenes depends upon a two cell system in which the donor lymphoid cells provide the immunological reactivity to the organism and recipient macrophages provide the mechanism through which resistance is expressed. Accompanying papers provide additional support for this view, and reasons for believing that delayed-type hypersensitivity and acquired cellular resistance are mediated by the same population of immunologically committed lymphoid cells.