3-Cl-AHPC inhibition of leukemia and CFU-GM colony formation. (A) Leukemic colonies fewer than 20 cells (•), between 20 and 40 cells (○), and more than 40 cells (▾). Mean numbers of colonies in control cultures were as follows: Patient 1 (Pt 1), 205 ± 25, fewer than 20 cells; 100 ± 10, 20 to 40 cells; 45 ± 5, more than 40 cells. Patient 12 (Pt12), 370 ± 60, fewer than 20 cells; 125 ± 10, 20 to 40 cells; 50 ± 10, more than 40 cells. Patient 13 (Pt 13), 285 ± 20, fewer than 20 cells; 85 ± 10, 20 to 40 cells; 40 ± 5, more than 40 cells. (B) CFU-GM colonies were grown and their formation was assessed as described in “Materials and methods” in the presence and absence of varying concentrations of 3-Cl-AHPC. The results represent the mean and standard deviation of 3 separate experiments using different normal marrow and patient leukemia samples.