Myeloid growth activity of rheumatoid arthritis (RA) and normal serum was measured with a newly developed simple method. Abnormally high titers of this activity were found in the bone marrow blood serum adjacent to joints with RA where abnormal myelopoiesis had been found. Maximal activity for myeloid growth was found at a molecular weight of 70 kDa in gel filtration of serum, different from the size of any known factors involved with myelopoiesis.