Somatomedin-A (Sm-A) bioactivity in serum and amniotic fluid as related to weight in the fetal pig.

Serum and amniotic fluid Sm-A bioactivity has been assayed in the fetal pig using a double labelling technique. 3H-thymidine activity was always higher in fetuses and lower in sows than 35S activity. 35SO4 uptake was not stimulated or was inhibited in about 30 p. 100 of the fetal sera. 3H-thymidine uptake was stimulated in all of them. There was a significant correlation (P less than 0.001) between fetal weight and serum Sm, as judged with both 35SO4 and 3H-thymidine. No correlation was found between fetal weight and amniotic fluid Sm, between amniotic fluid and serum Sm or between 35SO4 and 3H activities. It is concluded that the latter activities were due to two or more factors instead of only one.