Growth-stimulating activities toward HEL cells in various culture supernatants. Conditioned media (CM) were harvested from one day cocultures of HEL cells and HESS-5 cells. The supernatant of a culture of HESS-5 cells alone (HESS-5 CM), or a coculture of HEL cells and HESS-5 cells with direct cell contact (HESS-5 + HEL CM) or without cell contact, the latter being prevented with a microporous membrane (HESS-5 + HEL, filter-separated CM), was added to HEL cells (5 × 10³ cells each) at the final concentration of 25% (vol/vol) in triplicate wells. The assay used for³H-thymidine incorporation by HEL cells was described under Materials and Methods. Background indicates a culture of HEL cells alone in RPMI-1640 medium supplemented with 0.1% (wt/vol) BSA as a negative control. HESS-5 + HEL, coculture indicates a coculture of HEL cells and HESS-5 cells with direct cell contact as a positive control. The results are expressed as mean values ± SD of the mean.