BACKGROUND The cytogenetic examination of the bone marrow (BM) depends on the presence of sufficient amount of mitoses on slides prepared for analysis. The quantity and the quality of mitoses found in BM cultivations of patients with MDS is rather low and a new possibilities to increase the proliferative activity of the cells are under research. One promising method is the stimulation of the BM cells with addition of hemopoietic growth factors. The aim of this study was to evaluate in vitro influence of GM-CSF on the proliferative activity and the representation of chromosomally different clones in the patients with MDS. RESULTS The bone marrow cells of 25 patients (12 males, 13 females) of mean age 52.5 years (range 20-85) were examined. The mitotic index (MI), quality of mitoses and chromosomal complement were evaluated in 24 and 72 h cultivations with 100 and 1000 ng/ml GM-CSF concentrations. No stimulation was used in control cultivations. After 24 h of cultivation 100 ng/ml of GM-CSF increased the mitotic index (112%) but lowered the quality of mitoses (-7.6%). On contrary after 72 h of presence of 100 ng/ml GM-CSF both parameters showed the decrease (MI-30%, quality of mitoses-36%). Similar effect had the addition of 1000 ng/ml for 24 h of cultivation (MI-5%, quality of mitoses-11%). Twelve patients had normal karyotype on all studied cells in every type of cultivation. The ratio of chromosomally abnormal clones was the same in six other cases and only non-significant changes appeared in seven patients after GM-CSF stimulation. CONCLUSIONS The influence of GM-CSF on the division of the bone marrow cells is strictly individual. GM-CSF in concentration of 100 ng/ml in 24 h cultivation increases the activity of proliferation, but at the same time moderately decreases the quality of mitoses. The presence of GM-CSF has not changed the cytogenetic results.