PURPOSE: To obtain the subject reducing agent containing a cashew nut husk oil and an anacardic acid as active ingredients, permitting the multiplication of coccidial protozoa while sufficiently giving an immunological stimulation, and simultaneously decreasing damages to poultries by reducing lesions.

CONSTITUTION: This reducing agent contains a cashew nut husk oil and/or an anacardic acid as active ingredients. The cashew nut husk oil is obtained e.g. by heating husky cashew nut to separate the husks from the nuts, heating the husks as such, and subsequently separating the oil from the solid content. The anacardic acid is obtained by subjecting the cashew nut husks e.g. to silica gel chromatography. When the reducing agent is added to a feed and subsequently administered to poultries, the active ingredient is preferably added to the feed in an amount of 0.1-1wt.%.