Colibacillosis represents a major cause of diarrhea in young rabbits. In order to elucidate protective effect of milk, in vitro and in vivo experiments were carried out. In the in vitro experiment, rabbit milk treated with lipase significantly decreased the number of viable cells in cultures of Escherichia coli, O128 serotype, from 10.3 to 6.2-7.3log(10)/(cfuml). The lipase effect was the same with heat-treated (100 degrees C/10min) and raw milk. Raw milk without lipase decreased the number of E. coli only marginally. In the in vivo experiment, weaned rabbits received feed contaminated with the same bacterium. The course of the infection was moderate, only 2 out of 36 infected rabbits died. Seven days after inoculation, caprylic acid at 5g/kg feed and triacylglycerols of caprylic and capric acid at 10g/kg feed decreased faecal output of E. coli from 10.2log(10)/(cfug) to 5.8 and 6.1log(10)/(cfug), respectively. The number of E. coli in faeces of non-infected rabbits averaged 4.0log(10)/(cfug). The growth of infected rabbits was slow for 2 weeks after infection. In the third week a compensatory growth was apparent. At the end of the experiment, average body weights of rabbits receiving caprylic acid and those of non-infected rabbits were not significantly different. It can be concluded that (i) lipids rather than proteins seem to be responsible for the antimicrobial activity of rabbit milk; and (ii) this activity was lipase-dependent. Caprylic acid or oils with high a concentration of it may be used as feed supplements for weanlings.