Peripheral blood from normal humans and patients with circulating atypical mononuclear cells have been studied for their ability to form granulocytic colonies in vitro in the agar-gel method of hematopoietic cell culture. WBC from patients with chronic lymphocytic leukemia failed to proliferate. Normal human white blood cells gave rise to 0.5-8 colonies per 1 x 106 cells plated, while peripheral blood containing large mononuclear cells gave rise to 15 to 120 colonies per 1 x 106 cells plated, depending upon the number of such cells plated. Colonies averaged 500 cells in size by day 20 of culture and appeared to be granulocytic.