Immunological and biochemical characteristics of a 100,000 MW biglycan-like haemopoietic factor, purified from thymic myoid cells 871207B, were studied to distinguish them from macrophage colony-stimulating factor (M-CSF), which they resemble in activity and biochemical properties. Rabbit antibody raised against a synthetic peptide fragment (J-1) designed from amino acid sequences specific to the 100,000 MW factor responded to 871207B cells, the conditioned medium of 871207B, and capillary-like structures in the thymus, but not to M-CSF producer L-929 cells or the conditioned medium of L-929 cells. In contrast, M-CSF epitope was detected in L-929 cells and the conditioned medium cells but not in 871207B cells or the conditioned medium, even after enzymatic digestion of glycosaminoglycan chains. Treatment of the 100,000 MW factor with chondroitinase ABC and AC produced a 50,000 MW component. Digestion of this product with N-glycanase resulted in a 40,000 MW protein component. These results suggest that the 100,000 MW factor is a proteoglycan consisting of a core protein with an apparent molecular mass of 40,000 MW, a 50,000 MW chondroitin sulphate chain and 10,000 MW N-linked oligosaccharide chains. A small amount of a 40,000 MW monocytic cell growth activity was also found in the 871207B cell-conditioned medium. An enzymatically obtained 40,000 MW factor, the conditioned medium 40,000 MW factor, and the 100,000 MW factor were specifically eluated from an anti-J-1 IgG-immobilized affinity column with monocytic cell growth activity, suggesting that the biological activity resides in the 40,000 MW core protein. The 100,000 MW factor induced the proliferation and differentiation of monocytic lineage cells from a variety of sources, such as bone marrow cells, peritoneal exudated cells and brain microglia cells.