We have used crossed Immunoelectrophoresis to identify and establish the relative amounts of serum proteins secreted by a differentiated cell line (Fao) derived from a Reuber H35 rat hepatoma. Our results show that these cells secrete at least 15 plasma proteins. Ten of these: albumin, α1-antitrypsin, α1-lipoprotein, α1-macroglobulin, α1-antichymotrypsin, GC-globulin (transcalciferin), fibronectin, hemopexin, transferrin and the C3 component of complement have been identified. To examine the feasibility of using the Fao cell line as a model for studies on the regulation of hepatic protein secretion, we measured the relative amounts of 10 serum proteins secreted into the growth medium after exposure of these cells to dibutyryl cyclic AMP, hydrocortisone and a combination of both compounds. We also examined the effects of growth temperature (33.5°, 37° and 39°C) and the removal of fetal calf serum from the growth medium on the relative amounts of these proteins secreted. We found that the rates of secretion of most of the serum proteins were altered by one or more of the treatments used in these experiments. In addition, detectable levels of secretion of three serum proteins, fibronectin and two unidentified, occurred only under certain of the experimental conditions. These results demonstrate that the pattern of proteins secreted from Fao cells can be experimentally altered and indicate that this cell line may be a useful model for studies on the control of hepatic protein secretion.