The effect of different serum sources on the growth of human bone marrow mononuclear cells in liquid culture was investigated. Newborn calf serum failed to support growth in liquid cultures whether or not exogenous colony-stimulating factor was present. Neither adherent nor nonadherent cells proliferated in medium supplemented with horse serum. Fetal calf serum allowed proliferation of the nonadherent cell population, but only in the presence of colony-stimulating factor. However, no growth of adherent cells was observed in these cultures. Both the nonadherent and adherent populations grew well in the presence of pooled human sera. While growth of the nonadherent population was minimal in the absence of colony-stimulating factor, the adherent population appeared to increase to a greater extent when the factor was absent. The positive effect of the human serum was shown to be dose dependent in that as the proportion of serum was increased in the medium, the number of cells recovered from the cultures increased, regardless of the presence or absence of exogenous colony-stimulating factor.