Because of the variability of different placentas to condition culture medium for colony-stimulating factor (CSF), augmentation of CSF elaboration using endotoxin and BCG during the incubation period was attempted. Of 14 batches of human placental conditioned media (HPCM) prepared from different placentas, 6 were inactive (no colony formation at concentrations of 5-40% vol/vol), 5 were a weak stimulator of colony formation (producing < 10 colonies/105 human marrow cells plated at a HPCM concentration of 30%) and 3 were active (producing 30 or more colonies/105 cells plated at HPCM concentration of 30%). In 6 cases augmentation of CSF elaboration was attempted using endotoxin or BCG and in all a marked increase in CSF was achieved. The increased colony formation was due to greater increase in neutrophil and macrophage colonies rather than eosinophil colonies. The method of augmenting CSF elaboration in HPCM could have important implications for producing large quantities of potent and standardized source of CSF for the use in clinical laboratories and for increasing the yield of CSF for further purification.