By the method of scanning electron microscopy (SEM), the inner mucosal surface of the ileum, ceacum and colon was studied in inbred BALB/c mice. Two-day-old mice were infected with either 10(6) oocysts of Cryptosporidium parvum and 10(8) CFU of porcine and human strains of the bacterium Campylobacter jejuni or with a combination of both enteropathogens. Pathological changes in infection with C. parvum were related to enterocytes and villous atrophy appeared. In infection with C. jejuni, pathological changes were related to goblet cells. In combined infections, pathological changes were similar to those in monoinfections and occurred simultaneously within the intestine. Synergistic interaction of C. parvum and C. jejuni manifested itself morphologically in a more intense colonization of the inner surface of the small and large intestine by C. jejuni, in a more intense infection of the caecum and colon by C. parvum, and in prolongation of severe, massive infection of the small and large intestine, and also a prolongation of the patent period.