A new stabilized composition comprising colistin sulfate as active ingredient is now provided, which is composed of a mixture of the micro-particles of colistin sulfate and micro-particles of beta -starch and/or soybean meal as dispersed in the continuous phase of alpha -starch, and which is in the form of composite particles of said mixture having particular particle size range. The colistin sulfate component of this composition is stabilized from being degraded and inactivated to an unallowably low level, even when this composition has been mixed with a blended feedstuff, followed by being shaped into pellet form by compression at a high temperature of 120 DEG C in the presence of hot water vapor.
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