We characterized the molecular species of human macrophage colony- stimulating factor (hM-CSF) found in serum and urine, using immunoblot analysis after partial purification on an antibody-bound affinity column. Although antibodies were prepared using the recombinant product of the large form of hM-CSF with a molecular weight (MW) of 85 Kd as the antigen, this immunoblot system was also capable of detecting the small form of hM-CSF with a MW of 40 to 60 Kd. A single band with a MW of 43 Kd, which reacted with anti-recombinant hM-CSF IgG but not with control IgG, was found when serum and urine from normal adults underwent electrophoresis on reduced sodium dodecyl sulfate- polyacrylamide gel and subsequent immunoblotting. This band represented a subunit of the large form of hM-CSF, because the large form of hM-CSF is a homodimer of a subunit with a MW of 43 Kd and the small form of hM- CSF is a homodimer of a subunit with a MW of 20 to 30 Kd. Analysis of serum and urine from leukemic patients and pregnant women, who had higher serum levels of hM-CSF than normal adults, showed only a single band with a MW of 43 Kd as a hM-CSF-specific molecule. These results suggest that the large form of hM-CSF is the major species in human body fluids.