Summary:After mezerein treatment of suspension cultures of the acute monocytic human leukemia cell line THP‐1, cells became adherent to plastic culture surfaces, lost division potential, acquired Fcreceptors, displayed phagocytic activity, and expressed increased nonspecific esterase staining. Serum‐free RPMI 1640 medium conditioned by adherent THP‐1 cells was examined for the presence of biological response modifiers. Preparative isoelectrofocusing of concentrated medium in the presence of various pH gradients of Ampholine ampholytes resulted in the separation of the following activities: fibroblast growth‐stimulating activities in pH ranges of 4.10‐4.55 and 5.30‐5.45; colony‐stimulating factor (CSF) for mouse bone marrow cells at pH 4.10‐4.55; and a malignant cell growth inhibitor comigrating with a lymphocyte‐activating factor (interleukin‐1) at pH 6.70‐6.95. Both CSF and the fibroblast growth stimulator isofocused at pH 4.10‐4.55 coeluted following molecular‐sieve chromatography through P‐100. CSF‐induced colonies were composed of nongranulocytic mononuclear cells. Chromatography through an ACA‐54 column separated interleukin‐1 from most of the growthinhibitory activity.