To examine whether sarcoid epithelioid cells of granulomatous lesions produce colony-stimulating factor (CSF), a growth factor of monocyte-macrophage series, the primary culture of sarcoid epithelioid cells was carried out from 16 biopsied lymph nodes which contained sarcoid granulomas, CSF activity was demonstrated at high titer in the conditioned media from all the epithelioid cell cultures. The conditioned medium of the epithelioid cell culture was harvested, concentrated by Amicon YM-10 membrane, and loaded on a Sephadex G-75 column. Two distinct peaks of CSF activity were noted just before and after bovine serum albumin region. The molecular weights were estimated at about 75, 000 and 36, 000, respectively. Both CSF fractions stimulated colony formation by murine bone marrow cells. The lower molecular weight CSF was active on human bone marrow cells, but the higher molecular weight CSF did not stimulate colony formation by human bone marrow cells.