Phytohemagglutinin-stimulated peripheral blood mononuclear cells (PBMC) were treated with increasing concentrations of filgrastim, the unglycosylated methionine granulocyte colony-stimulating factor of man (rhG-CSF), and cultured for 72 h. There were no impaired proliferation or differentiation of proliferating PBMC, no impaired expression of activation markers such as the low-affinity interleukin 2 receptor and transferrin receptor, and no induction of sister chromatid exchanges. Under these conditions, no effects of a general DNA destabilization of peripheral blood leukocytes was observed. Thus, longterm administration of therapeutical concentrations of rhG-CSF should not produce severe mutagenic effects.