A methylcellulose clonal assay was used to quantify clonogenic capacity of primitive cells obtained from BM, UCB and HL-60 lineage. Clonogenic capacity of BM was increased, showing that H2O2 induced the improvement of the capability of these cells to generate their progeny. UCB colonies decreased around 70% while HL-60 stimulated with H2O2 was not able to generate clones. The data are expressed as the mean ± SEM of 4 experiments. *P< 0.05, ANOVA.