High levels of plasma erythropoiesis stimulating factor(s) (ESF) have been found in neonatal WLOmice during rapid growth. A previous study on hypertransfused neonatal animals indicated that the high ESF could not be due to the concomitant postnatal anemia alone. The present investigation was performed to answer the question: Is the high plasma ESF in neonatal WLO-mice erythropoietin (Ep) alone, or Ep in combination with other factors? The ESF of plasma from 20-day-old animals and standard Ep were compared in a cell culture assay for ESF based on erythroid colony formation, and also by means of gel filtration chromatography and affinity chromatography. Nonfractionated plasma and standard Ep showed parallel dose response curves and additive activity in the ESF assay. After gel filtration the detectable ESF of plasma was eluted in the same position as that of standard Ep, corresponding to an estimated molecular weight range of 34-65000 daltons. The ESF of intact plasma, fractionated plasma, and standard Ep were identically bound to and eluted from the affinity chromatography column. These results show that the ESF of plasma from 20-day-old animals can neither be separated into several factors, nor distinguished from that of standard Ep by the methods used. It is therefore concluded that the high plasma ESF found in neonatal WLOmice probably consists of Ep alone.