To achieve a comparable protein and cell production by culturing of BHK-21 cells in monolayer, in a medium containing adult bovine serum (ABS) and in a medium containing fetal bovine serum (FBS), 1.5 times (v/v) more ABS than FBS has to be added. At a volume fraction in the medium of less than 2% ABS, no proliferation is observed. For a fixed protein and cell production, the amount of required protein in a medium containing ABS can be reduced by addition of a protein extract from pituitary glands from calves (PEP). The relative ceil density after 7 days in a culture medium containing 2 or 5% (v/v) ABS is 3 times higher when 0.2 g/liter pituitary extract is added. This indicates a synergistic interaction between components from ABS and from the protein extract. The addition of a pituitary extract to a medium with ABS also induces a prolonged cell proliferation period. A combined addition to a medium with ABS of 0.02 g/liter PEP and 0.15 g/liter heparin yields almost the same cell density as the addition of 0.2 g/liter PEP alone, indicating the presence of fibroblast growth factor (FGF) in the pituitary extract. However, the enhanced cell production by the addition of a pituitary extract to a medium containing ABS cannot be explained only by the presence in this crude extract of a-or bFGE Abbreviations: ABS -adult bovine serum; BE -protein extract from whole bovine brains; FBS -fetal bovine serum; PEP -protein extract from pituitary glands from calves; PEX -purchased bovine pituitary extract; PDT -population doubling time

Summary. The lipid concentration in adult bovine serum (ABS) is 7.93 + 1.29 mM and in fetal bovine serum (FBS) 2.90 + 0.39 mM. A high lipid concentration in the medium reduces the tested cell culture parameters. When a culture medium for BHK-21 cells, containing FBS, has the same lipid concentration as a medium containing the same volume fraction of ABS, the cell density and protein synthesis after 7 days, are reduced by about 15%. However, the cell density and the protein synthesis, in this medium with FBS and additional lipids, are still about twice as high as in a medium with ABS. The hydrophilic silicon particles Aerosyl have a specific affinity for lipids. Incubation in suspension of ABS with 2.5 g Aerosyl 300/liter serum, before use in a culture medium, yields a maximal increase of the cell density of BHK-2t cells and protein synthesis. However, after this incubation, the lipid concentration in ABS is still 2.40 + 0.71 times higher than in FBS. Some other materials (gels) remove lipids from ABS, but do not yield an increase of the cell density and protein synthesis, comparable with ABS treated with Aerosyl 300. These observations indicate that, besides lipids, Aerosyl 300 also removes other components from serum. Presumably some of these removed components inhibit cells in culture and others are essential for cells in culture. This is supported also by the observed maximal stimulation of cell in culture in a medium with FBS, treated with 2.5 g Aerosyl/liter serum.