A homologous series of α, α-trehalose 6, 6'-diesters involving five new compounds, acetate, octanoate, decanoate, laurate, and myristate, in addition to the known palmitate, stearate, and behenate were synthesized according to the pathway reported in our previous paper. These esters, except the clecanoate, were tested for their in vitro antitumor activity against two leukemia cell lines, L-5178 Y and L-1210, by the cell culture method and found to be. somewhat effective in inhibiting the growth of these tumor cells. In either case, the lowest ID50 value was observed with the laurate; the increasing or decreasing acyl chain-length gave higher ID50. values. When in vivo antitumor activity of the esters, with one exception of the behenate, was evaluated against mouse Ehrlich ascites carcinoma by the total packed cell volame method, it was found that the compounds having the acyl moieties longer than deca, _ noyl were, all highly effective, while the two shorter analogs, acetate and octanoate, were completely inactive. Chemical and Biochemical Studies on Carbohydrate Esters. XIV.