BACKGROUND: One of major challenge of cytogenetics is to obtain qualitative metaphases to achieve a meaningful analysis. Methotrexate (MTX) synchronization and the Giant Cell Tumor-Conditioned Medium (GCT-CM) have been used to improve the metaphase preparation from hematopoietic malignancies. The purpose of this study is to determine Mitotic Index (MI) of bone marrow samples under several culture conditions that may improve the quality of chromosome preparation. METHODS: Sixty nine bone marrow samples were cultured into 3 groups by traditional methods. The first group was tested for the effect of cell concentration on MI. The second, for the effect of MTX concentration on MI. The third group was classified into 4 subgroups as follows: 1) MTX only in 24 hour culture 2) MTX and GCT-CM in 24 hour culture 3) 48 hour culture without MTX 4) 48 hour culture and GCT-CM. MI was calculated as the ratio of metaphase to interphase cells in 1000 cells. Quality of metaphase was evaluated by classified the metaphase cell into 3 types. RESULTS: The first and second groups revealed no relationship between cell concentration, amount of MTX and MI, respectively. The third group showed significant differences among four subgroups. The MI increased in subgroups using GCT-CM and in the 48 hour culture, with greatest increase in group using 48 hour culture and GCT-CM simultaneously. CONCLUSIONS: The use of GCT-CM medium and prolonged culture improved the quality of metaphase cells and MI. It is therefore beneficial to use these conditions in cytogenetic studies on bone marrow in hematologic disease.