We describe a culture medium that does not require the addition of serum

in which erythropoietic bursts are produced from murine bone marrow cells with efficiencies at least as great as those of serum-containing media. Three ingredients were shown to be essential: bovine serum albumin

cholesterol

and eryth’ropoietin. The system supported burst formation without the addition of conditioed medium as source of burst-promoting factor(s). Filtration of cholesterol and delipidation of BSA both impaired the ability of the medium to support burst production. The "serum-free" medium described here is useful for erythropoietic culture: it is reliable

hi-ghtly efficient

economical

and simple to prepare from ingredients that require no pretesting and are readily available from commercial sources.