The effect of fetal bovine serum (FBS) on the secretion of apolipoprotein A-I (apo A-I) by HepG2 cells was studied. The cells incubated with FBS always secreted more apo A-I than the cells incubated with serum-free medium. The changes in the rate of apo A-I secretion were observed within 1 h after addition or depletion of serum. The high-density lipoproteins (HDL) or the lipoprotein-deficient serum (LPDS) obtained from FBS also stimulated apo A-I secretion rapidly to the same level as obtained with FBS. Addition of low-density lipoproteins did not have any effect. The rate of general protein synthesis was not affected by short-term incubations with or without serum or HDL. The rate of apolipoprotein E secretion by these cells did not change significantly, parallel to the changes in apo A-I secretion in the presence or absence of FBS. It is concluded that serum may have a factor that plays a specific role in the regulation of apo A-I secretion by the liver cells and this factor is associated with the HDL fraction.