Stimulation of the in vitro immune response by factors present in the human serum

Two distinct factors which stimulate the recovery of plaque-forming cells in the vitro immune response have been found in human serum. One acts in the presence of fetuin and the other augments the recovery of plaque-forming cells in its absence. The molecular weight of the former is higher than 1 X 10(5). The two factors could be separated from each other by ammonium sulphate fractionation. Large differences were found in the amount of these factors in sera obtained from different human donors.