We have studied the effects of dialyzable lymph node extracts (DLNE), from bovines immune or nonresponsive to tuberculin, or human cell-mediated immunity to purified protein derivative of tuberculin (PPD) in vitro. The effects of bovine DLNE (B-DLNE) on the immune responsiveness of human cells were evaluated using both the direct and indirect agarose leukocyte migration inhibition (LMI) assays to measure the migration of human leukocytes, and by determining the effects of B-DLNE on the rate of regeneration of receptors for sheep red blood cells (SRBC) on human thymus-derived (T) lymphocytes after trypsinization. The effects of human dialyzable leukocyte extracts (H-DLE) were also evaluated in both systems, as controls. The results indicated that both B-DLNE and H-DLE contain a “transfer factor-like” activity which can be detected in vitro with the agarose LMI assay. Both B-DLNE and H-DLE contained components which promoted measurable specific antigen responsiveness in previously nonimmune lymphocytes; specificity was shown by analyses of the migration of human leukocytes in the presence of PPD, coccidioidin, or candida antigen in the presence and absence of H-DLE or B-DLNE from donors specifically responsive or nonresponsive to the various antigens. The components promoting specific antigen responsiveness were dialyzable and were derived only from lymphoid tissues. Dialysates from brain, liver, or skin failed to promote similar effects. When the amount of B-DLNE tested was less than that found to be optimal for promotion of antigen-specific responsiveness, pronounced enhancement of migration of human polymorphonuclear neutrophils (PMN) was seen, similar to the results obtained previously with H-DLE. Both B-DLNE and H-DLE significantly increased the rate of SRBC receptor generation on human T cells (up to a threefold increase at optimum concentrations). This activity, termed “mononuclear leukocyte-derived maturation factor” (LDMF), was found preferentially in lysates of lymphoid or reticuloendothelial organs and cells.