Colony-forming cells (CFC) and colony-forming ability in peripheral blood and bone marrow were studied in 22 patients with different stages of sarcoidosis, with or without the addition of different Kveim preparations. The study showed no statistical differences between the colony growth in healthy persons compared to patients with sarcoidosis in stages I and II. The counts in these groups were, however, lower than in patients with sarcoidosis in stage III (P < 0.01). The lowest counts of CFC were found in the highly active cases suffering from fever, joint pains and erythema nodosum, and these counts were also lower than generally found in normal cases. After stimulation with mononuclear cells from healthy controls the number of colonies in the acute stages, however, reached normal values. This suggests either a low autostimulation or a neutralization of an inhibiting factor in the blood. It is suggested that if an inhibiting factor is present it is principally localized to the sarcoid granuloma. A pronounced inhibition of colony growth was seen when the Kveim membrane fraction prepared from sarcoid lymph nodes and spleen, was added to the cell cultures.