Adherent murine stromal cells support long-term in vitro lymphopoiesis or myelopoiesis dependent on the culture conditions used. A cell line, TC-1, isolated from long-term liquid murine marrow cultures under conditions approaching those permissive for lymphoid growth, has been found to produce an activity that acts synergistically with interleukin-3 (IL-3) or colony-stimulating factor-1 (CSF-1) to stimulate in vitro myeloid colonies, but which has no intrinsic colony-stimulating activity. We report here the presence of multiple growth factors in conditioned medium (CM) from the TC-1 line, including granulocyte-macrophage colony-stimulating factor (GM-CSF) (bioassay with antibody blocking and messenger RNA [mRNA] analysis), granulocyte CSF (G-CSF) and IL-4 (factor-dependent cell line bioassay), and CSF-1 (radioimmunoassay, mRNA) along with a pre-B cell inducing activity, which appears separate from these CSFs and segregates with the myeloid synergizing activity through anion exchange, sizing, and Conconavalin A chromatography. Because these activities are not yet purified to homogeneity, their identity or lack of identity remains an open question. Assays of TC-1 CM or cellular mRNA analysis have given negative results for IL-1, IL-2, IL-3, IL-6, and IL-7, and IL-6 does not stimulate pre-B cells in this assay. However, IL-4 and G-CSF do stimulate in vitro induction of pre-B cells from pre-B and B-cell-depleted Balb/C marrow and are present in CM by selective cell line assay. A monoclonal antibody to IL-4 that inhibited its pre-B inducing activity did not inhibit pre-B inducing activity of TC-1 CM. These data suggest the existence of a unique synergizing and pre-B inducing factor(s) in TC-1 CM. Given the known capacity of subliminal levels of growth factors to act synergistically, an alternate possibility is that these biologic phenomena represent the actions of low concentrations of growth factors acting synergistically and possibly associated with some core protein.