An anti-diabetic composition comprising an effective amount of at least one member of LPS whose macrophage activation ED₅₀ is 0.4 - 100 ng/ml of culture solution in terms of its limulus test-positive LPS content observed on a sigmoid curve prepared by determining the ability of the LPS to activate the TNF productivity of macrophage cultured in vitro, and plotting the macrophage activation abillty (%) along the axis of ordinate wherein the ability is estimated to be 0 % in the case where it corresponds to the quantity of TNF produced by macrophage with no LPS added thereto, and 100 % is assigned to the macrophage activation ability which provides the maximal and constant quantity of TNF produced by the macrophage and plotting the limulus test-positive LPS content of the LPS along the axis of abscissa on a logarithmic scale, in admixture with a pharmaceutically or veterinarily acceptable carrier, such that when administered to an animal, diabetes mellitus of said animal is prevented or cured; and
   a method of treating diabetes mellitus of an animal comprising administration to said animal an amount of the above composition effective to prevent or cure the diabetes mellitus of said animal.