Titers of [human] erythroid regulatory factors (ERF), as measured in vivo with a standard erythrocythemic mouse technique, showed a highly significant correlation with those obtained in vitro wtih a fetal mouse liver cell (FMLC) assay. The results from the assay in vitro were somewhat higher than would be predicted from the assay in vivo. The FMLC technique was a valid and useful technique particularly in the study of serum ERF titers.