With a sensitive technique for the identification of lymphocytes and lymphoblasts bearing the surface antigens ϑ, TL, and immunoglobulin, the induction of ϑ-positive small lymphocytes in bone marrow could be observed. Dibutyryl cyclic adenosine 3′,5′-monophosphate and thymosin were both effective in inducing precursor cells to express ϑ antigen. Concanavalin A and bacterial lipopolysaccharide were also shown to be inducers. After treatment with concanavalin A but not with lipopolysaccharide, lymphoblasts developed in marrow cultures which were positive for ϑ and TL antigens; when spleen was cultured with concanavalin A, lymphoblasts were ϑ-positive and TL-negative. It is concluded that the cell responsive to concanavalin A (and phytohemagglutinin) in bone marrow is a ϑ-negative precursor (stem) cell, which is induced first to express ϑ and TL antigens, and then to differentiate to a lymphoblast.