Medium conditioned by human placental tissue stimulated granulocytic and monocytic colony formation by human marrow cells in semisolid agar cultures. The colony-stimulating activity of unfractionated conditioned medium was equivalent to the activity of standard peripheral blood underlayers. Placentas were a reliable source of active material, and 1 placenta provided enough material to stimulate 5000-10,000 cultures of normal or leukemic cells. The colony-stimulating factor in human placental conditioned medium (CSFHPCM) was concentrated and purified 1800-fold using ammonium sulfate precipitation, calcium phosphate gel absorption, DEAE-cellulose batch absorption, gel filtration on Sephadex G-150 and polyacrylamide gel electrophoresis. The active factor behaved on gel filtration as a macromolecule with an apparent MW of 30,000 daltons. The active factor in placental conditioned medium was not dependent on the presence of adherent marrow cells with endogenous colony-stimulating activity.