We are engaged in the purification of a factor released in tissue culture by thymocytes, and probably by T-cells in the spleen, which enhances antibody synthesis against any antigen present. The factor is released by specific antigen acting on previously primed cells, and by allogeneic interactions among cells. It is apparently a protein, or attached to a protein, of about 80,000 daltons. We are also studying the binding of an antigen, beta-galactosidase, by lymphoid cells from normal animals. This binding, though demonstrably specific by inhibition experiments, does not change with age, being as extensive in new born or germ-free animals as in young adults.