Thirty patients with a variety of non-lymphocytic neoplasias were studied cytogenetically using short-term liquid cultures of bone marrow or peripheral blood cells with or without either granulocyte colony-stimulating factor (G-CSF) or granulocyte-macrophage CSF (GM-CSF). The final concentration of these growth factors was 10 ng/ml and the duration of culture was 48 h, since these provided optimal increases in mitotic index (MI). In GM-CSF-stimulated culture, 23 out of the 30 patients (77%) had a significantly (p < 0.05) higher MI than that in unstimulated culture, whereas only five (17%) did so in G-CSF-stimulated culture. The quality of banded chromosomes was considerably good in 17 out of the 30 patients (57%) with GM-CSF, whereas it was so only in two (7%) and three (10%) patients with no CSF and G-CSF, respectively. Of the 30 patients, 27 (90%) had the same chromosomal findings with G-CSF/GM-CSF as without CSF, but the remaining three (10%) showed a remarkable change after stimulation. Culture for 72 h with G-CSF or GM-CSF disclosed a minor abnormal clone which was undetected in 24 and 48 h cultures in a patient with myelofibrosis with myeloid metaplasia. Thus, compared with G-CSF, adding GM-CSF to cell culture may be useful for cytogenetic studies of non-lymphocytic neoplasias. The chromosomal findings may, however, be changed by these growth factors for some patients.