In order to observe the influences of chloroquine on the bone marrow thrombopoies, clinical tissue culture of bone marrow from normal guinea pigs was conducted. Chloroquine diphosphate sulutions at various concentrations were added directly to the culture media and observations were carried out to see effect of chloroquine on the functions of megakaryocytes of bone marrow. As a result, it has been seen that the thrombopoietic functions of bone marrow were hardly inhibited in the medium containning 2γ/dl and 20γ/dl of chloroquine, but were markedly inhibited in the medium containning 200γ/dl and 2000γ/dl of chloroquine.