Protective host immunity and delayed hypersensitivity were transferred to nonimmune ICR and C3H/HeJ mice with transfer factor prepared from the splenic lymphocytes of ICR Swiss mice immune to Salmonella typhimurium. Only mice injected with the "immune" dialysate exhibited significant footpad swelling (P less than 0.01) to a spent medium antigen of S. typhimurium, but not Listeria monocytogenes. Host survival to a lethal Salmonella challenge infection was seen only in transfer factor-injected mice. Also, these challenged animals had fewer numbers of bacteria present in their spleens (P less than 0.01) than did challenged mice previously injected with either control dialysates or commercial endotoxin. Neither the Salmonella antigens nor endotoxin present in the sterile transfer factor preparation was responsible for the transfer of host protection and delayed hypersensitivity, since none of the control dialysates resulted in any positive responses when injected into either the ICR Swiss or endotoxin-resistant C3H/HeJ mice.