(a) Fibroblast monolayers were grown in 10% fetal calf serum, and on day 6 after plating (day 0) the medium was replaced with medium containing 5% human serum from which lipoproteins had been removed. At the times indicated, extracts were prepared, and HMG CoA reductase activity was measured. (b) 24 hours after changing to medium containing lipoprotein-deficient serum, human LDL was added to give the indicated cholesterol concentrations, and HMG CoA reductase activity was measured in cell-free extracts at the indicated times (Reprinted from ref. 13).