Thiabendazole (TBZ), when given alone, stimulates hyperplasia of subcapsular thymocytes and of germinal center cells in normal mice. When given with the thymus-dependent neoantigen dinitrofluorobenzene, TBZ stimulates all compartments of the lymphohemopoietic system, including pluripo-tent hemopoietic stem cells (CFU-S Spleen colony-forming unit cells), presumptive prothymocytes (TdT-positive bone marrow cells), extramedullary hemopoiesis, and peripheral T and B cells. All of these effects appear to be mediated by a humoral factor, or factors, that accumulates in the serum between 1 and 6 h after treatment. Normal syngeneic recipients of 0.25—0.5 ml of this serum show changes in their lymphohemopoietic systems 3 to 8 days later. Serum fractionation on Sephadex G-25 indicates that the transferred activity is not due to residual TBZ or its major metabolites, but rather to an induced factor. We propose that this factor results from the interaction of stimulated macrophages and T cells. Pending further characterization, we have designated this factor LHSF, lymphohemopoiesis stimulating factor.