Effects of the concentration of FBS or rabbit serum on the prevention of calcification in cell culture. Confluent cells were exposed to various concentrations of FBS or rabbit sera for 2 hrs. Concentrations of the sera ranging from 0.004 to 1% were used. The initial Ca × P ion product in culture media was 9.37 mM2. Each data point for Ca and P determinations represents the average of duplicate data. The C1/2 values in these experiments were about 0.07 % for Ca and P deposition. The serum levels are expressed as a negative log scale of the percentage of the serum, i.e., the higher the scale to the right of the X axis, the more the serum was diluted as indicated. The deposition of Ca and P was determined by colorimetric procedures and expressed as μmol Ca or P per mg of cell proteins. At 0.004% serum, the amounts of mineral Ca and P were nearly equal to those without serum. (A) FBS. (B) Normal rabbit serum.