Human fibroblast conditioned media contains growth‐promoting activities for low density cells

Normal diploid human fibroblasts, cultured at high density (1–2 × 10 5 cells per cm 2 ) release two growth promoting activities into the culture medium. The fibroblast proliferation activity‐conditioned medium facilitates the attachment of low density cells to the substrate. That activity resides in a non‐dialyzable material that is sensitive to proteolytic inactivation. A second activity is dialyzable and can be recovered in the dialysate. In the presence of serum it stimulates cell growth. After 168 hours of incubation conditioned medium cultures contain five times more cells than are present in comparable cultures without conditioned medium. A reproducible biological assay for each activity is described.