Abstract: Proteinaceous antibiotics produced by ruminal bacteria are provided. The diverse group of ruminal bacteria known as Butyrivibrio spp. is a preferred source of such proteinaceous antibiotics. The proteinaceous antibiotics are generally resistant to gastric proteases, exhibit a high level of hydrophobicity, are effective to inhibit growth of target organisms under anaerobic conditions, are ineffective in aerobic conditions, and have a molecular weight of less than about 5 kDa. Also provided are methods for identifying ruminal bacteria which produce such proteinaceous antibiotics, and methods for producing the proteinaceous antibiotics.