The colony and cluster-forming capacity of peripheral blood cells obtained from 25 patients who had chronic granulocytic leukemia (CGL) was evaluated in a double-layer agar culture. Increased numbers of colonies (mean, 264) and clusters (mean, 908) were obtained from the blood cells of nine of ten patients when first examined. The blood cells from 19 patients whose diseases were in the control phase formed normal or nearly normal numbers of colonies and clusters. During the aggressive phase, elevated blood colonies and clusters were observed in the 12 patients studied. Two to four times as many clusters as colonies were observed during these phases of CGL. At the time of blastic crisis, at least eightfold more blood clusters than colonies were observed in the eight patients studied. These studies indicate that the evaluation of blood colony and cluster-forming cells may provide a useful characterization of various clinical phases of CGL in individual patients.