The effects of anti-thyroid drug, methimazole (MMI), on haematopoiesis in inbred C57BL/6 mice were studied. The in vitro proliferative response of bone marrow (BM) cells to interleukin-3 (IL-3) was significantly increased when mice were provided with 0.1% MMI in water (w/v) ad librium for 4 to 6 weeks. Using soft agar agar colony assay, the numbers of myeloid cell colonies were also significantly increased in mice treated with MMI. However, the proliferative response of BM cells to IL-3 was found to be greatly reduced 10 weeks after MMI treatment. In vitro studies showed that MMI alone at the concentrations of 500 microM or above inhibited both the growth of normal BM cells in liquid culture and the formation of macrophage (M)-/granulocyte (G)-colonies in soft agar culture in a dose dependent manner. Direct cytotoxic effect of MMI (0 - 1250 microM) to normal BM cells was not observed. Results from this study suggested that MMI can modulate the development of myeloid haematopoietic cells.