The normal range of many chemical, physical, and biological (hormonal) properties of fetal bovine, calf, and newborn calf sera collected commercially are presented. We attempt to relate these properties to growth promotion in vitro. Whereas no definitive conclusions could be drawn from the data, high growth hormone levels, low endotoxin levels, and low levels of bilirubin seemed to have some association with good growth promotion if the levels of the other parameters were close to the mean. Large variations from the mean of any of the parameters seemed to affect the growth properties of the serum.