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Mice made tolerant to allogeneic tissues in neonatal life have been examined at different times for their ability to respond to the tolerizing determinants in a variety of assays (in vitro CML, MCL and in vivo GvH assays). All animals were tolerant in terms of their inability to produce CTL to the relevant determinants, and to induce GvH in lethally irradiated F1 recipients. Nevertheless, some mice also showed a normal MLC proliferative response and contained antigen-specific serum inhibitory factors, while other mice contained apparently antigen-specific suppressor cells. The pool of the latter, futhermore, was expanded considerably upon adoptive transfer of tolerant cells (with tolerizing antigens) to lethally irradiated syngeneic recipients. The data are compatible with the notion that suppression of clonal expansion represents the primary mechanism of tolerance maintenance (induction), and that the infrequently observed serum reactivity in such tolerant mice represents a vestige of the means whereby-cell mediated suppression was induced.

An assay system is described in which effector cells added along with suitable target cells inhibit, in a quantitative fashion, the subsequent uptake of 3H-thymidine by those target cells. Effector cells active in this assay, using embryonic fibroblast cells as targets, develop spontaneously in cultures of mouse lymphoid cells, but are apparently different from those described earlier by investigators of activity in cytotoxic assays. Further evidence is presented to show the development of spleen-derived effector cells with cytostatic activity (for embryonic fibroblast target cells) in mice during the course of normal pregnancy, or growth of spontaneously appearing mammary adenocarcinomas. Indeed, such effector cells can also be found within the growing solid mass itself. Different populations of tumour cells isolated from a solid tumour apparently differ in their susceptibility to growth inhibition by tumour-bearer-derived cytostatic effector cells, a phenomenon which may be related to metastatic spread of tumour cells.