Effects of methylmercury on the cell growth of mouse glioma in vitro were studied in relation to the microtubules as a possible target. The electron microscopic observation revealed that methylmercury added in culture medium specifically attacked microtubules in the cells at growth inhibitory concentration, while mercuric mercury injured microtubules to the same extent as other cell organelles. However the latter showed a little stronger depressing effect on tubulin polymerization in vitro than the former. Further, 3 other inorganic metal ions (Cd2+, Cu2+ and Cr3+), which were reported to suppress tubulin polymerization in vitro, did not disturb the microtubule networks even at their cell growth inhibitory concentrations. Therefore, of these metal compounds tested, when they were added in the culture medium, methylmercury seemed to specifically interact with microtubules and cause inhibition of cell growth.