AC133 antibody provides an alternative to CD34 for the selection and characterization of cells necessary for engraftment in transplant situations. We studied the effect of stem cell factor (SCF), interleukin 3 (IL-3) and interleukin 11 (IL-11) on the ex vivo expansion of human CD34+/AC133+ progenitors isolated from leukapheresis products from chemotherapy plus granulocyte-colony-stimulating factor (G-CSF) -mobilized adult donors. MiniMACS AC133+ isolated cells contained a mean of 85% (80-90) AC133+ cells. Enriched AC133+ cells co-expressed CD34+ 80%, CD71low 36.6% and CD33+ 6.6%. After a seven-day ex vivo expansion in the presence of SCF + IL-3 + IL-11, the number of cells increased 19 times. These cells expressed a mean 12% CD34+ and 74% CD71+ (23% CD 71high) and 59% CD33+. This means that the absolute number of CD34+ cells increased slightly, the number of CD33+ increased 100 times and cells with high density CD71high (23%) appeared. These cells represent the cells committed to erythroid differentiation. The increase in the number of CFU-GM with various combinations of cytokines SCF + Il-3 + IL-11 will be discussed. The number of CFU-GM in culture after a seven-day ex vivo expansion in the presence of SCF + IL-3 + IL-11 increased 45 times.