The effect of fetal bovine serum ultrafiltrate, containing low-molecular components (lower than 14 000 D), on the growth of cultures of the lymphoid Raji cells and fibroblasts BHK-21 was studied. The growth of the former did not differ from that in the control (in the medium with the whole fetal serum), while the latter did not proliferate in the medium with the ultrafiltrate. Thus, the growth of the lymphoid Raji cells and fibroblasts BHK-21 is controlled by different serum components. The Raji cells were exposed to an ultrafiltrate (up to 14 000 D) of the adult animal serum whose growth stimulating activity is known to be lower than that of the fetal serum. After the removal of components with molecular weights higher than 14 000 D from the adult animal serum, the growth stimulating activity of such a serum was seen increased, but not up to the level of the fetal serum. BHK-21 cells did not proliferate in the medium with this ultrafiltrate. It is proposed that the increase in the growth stimulating activity of the whole bovine serum in respect to the Raji cells after the removal of the components with high molecular weights may be due to the removal of lymphocyte growth inhibitors whose activity depends on the age of the animal serving a donor of serum.