CBA/Ca mice infected with the tetrathyridia of Mesocestoides corti were exposed to Schistosoma mansoni cercariae either simultaneously, or at varying intervals after the initial M. corti infection. Cohorts were infected with either parasite alone. The dual infected mice and the mice harbouring M. corti alone were significantly heavier than either naive controls or mice carrying the S. mansoni infection only. The livers and spleens from dual infection mice were also found to be significantly heavier than those from other groups. Free M. corti tetrathyridia were reduced in number in the peritoneal cavity of dual infected mice, as compared with mice harbouring a single infection. Furthermore, the intensity of the initial M. corti infection, as measured by the number of capsules surrounding parasites in the liver, was reduced when the mice experienced an S. mansoni infection 21 days later. The mice which were exposed to M. corti only exhibited more mast cells and eosinophils around encapsulated tetrathyridia in the liver than did dual infection mice, while cells surrounding S. mansoni egg granulomas in the liver were significantly increased in dual infection mice. An increase in serum alkaline phosphatase levels was detected in both the mice receiving the dual infection and the mice with the S. mansoni only infection, but not in mice harbouring M. corti alone.