Cattle that resisted experimental heartwater infection caused by the rickettsia Cowdria ruminantium produced significant levels of circulating alpha interferon (IFN-alpha), whereas animals that died from heartwater did not. In vitro, recombinant bovine IFN-alpha was found to significantly reduce the yield of Cowdria organisms in bovine endothelial cells, but even at a high concentration (1,000 U/ml), IFN-alpha did not completely prevent the growth of Cowdria organisms in these cells. This limited inhibitory effect of IFN-alpha is in agreement with the in vivo situation where an infectious process has to take place to induce a protective immune response. Our results suggest that IFN-alpha produced in vivo in response to Cowdria infection may represent an efficient way to slow down the infection and allow the animal to mount a protective immune response. IFN-alpha is the first endogenously produced factor shown to have anti-Cowdria activity.