In a search for alternatives of in-feed antibiotics, the antimicrobial activity of caprylic (C8:0), capric (C10:0) and oleic (C18:1) acid was investigated in pure cultures of 19 strains of rumen and rabbit caecal bacteria, and in incubations of the rumen and rabbit caecal contents. In glucose-grown bacterial cultures the minimum inhibitory concentration (MIC) of caprylic acid ranged from 1 to 3 µl∙ml^-1. Two strains of Bacteroides ovatus were less susceptible to capric than to caprylic acid. In other strains, the MIC of capric acid was 0.25-0.50 µl∙ml^-1. The growth of most strains was not much affected by oleic acid. An exception to this were rumen bacteria Butyrivibrio fibrisolvens (MIC from < 0.05 to 1 µl∙ml^-1) and Lachnospira multiparus ( MIC of 0.25 to 1 µl∙ml^-1). In incubations of the rumen and caecal contents caprylic and capric acid decreased the production of volatile fatty acids and gas, and increased production of lactate. In latter incubations the inhibitory effects of caprylic and capric acid were similar. In incubations of the rumen contents, capric acid was more efficient than caprylic acid when supplied at low concentrations (<1.25 µl∙ml^-1) , but less efficient when supplied at 2.5 and 5 µl∙ml^-1. Effects of oleic acid in rumen and caecal cultures were not significant, except the increase in production of lactate by rumen microorganisms. It can be concluded that microorganisms of the animal digestive tract are susceptible to inhibition by caprylic and capric acid added to microbial cultures at fairly low concentrations. Oleic acid was far less effective.