Resistance of outbred mice to subcutaneous challenge with S. abortus ovis was increased when previously or simultaneously parasitized with Eimeria Falciformis. This was measured by mortality rates and levels of splenic infection induced by Salmonella in parasitized and control NMRI or CD-1 (CR) mice. No effect was observed when E. falciformis was inoculated after S. abortus ovis.