The purpose of the present invention is to provide: a process for producing an immunostimulant for animals which is capable of improving the immunocompetence of animals the immunostimulant for animals and a feed for animals which includes the immunostimulant for animals. The process for producing an immunostimulant for animals of the present invention comprises a preceding-stage fermentation step in which Bacillus polymyxa is incubated in a culture medium containing a carbon source and a nitrogen source at a temperature higher than 25 DEG C but not higher than 35 DEG C, a late-stage fermentation step in which the culture obtained in the preceding-stage fermentation step is stored at a temperature of 25 DEG C or lower for 30 days or longer, and a molecular-weight regulation step in which the molecular weights of a polysaccharide and protein produced by the Bacillus polymyxa are regulated by adding an organic acid to the culture having undergone the late-stage fermentation step.本發明將提供能夠提升動物之免疫力的動物用免疫賦活劑的製造方法、動物用免疫賦活劑及使用此動物用免疫賦活劑之動物用飼料作為目的。本發明之動物用免疫賦活劑的製造方法,包括在包含碳源與氮源的培養基中,在超過25℃且35℃以下培養多黏芽孢桿菌(Bacillus polymyxa)的發酵前期步驟、將在前述發酵前期步驟中獲得的培養物於25℃以下保存30日以上的發酵後期步驟,以及藉由在前述發酵後期步驟後的培養物中添加有機酸,來調整經由前述多黏芽孢桿菌所生產之多醣類及蛋白質的分子量的分子量調整步驟。
