This study was performed to investigate the immunogenicity of autogenous E coli vaccines and their preventive effects on diarrhea in suckling piglets. Autogenous E coli live and killed vaccines were made from the E coli strains isolated from piglets showing diarrhea in field. In group I, pregnant sows were administered with live and killed vaccines at 4 and 2 weeks before parturition, respectively, Killed vaccines were administered twice to pregnant sows at 4 and 2 weeks before parturition in group II, and saline instead of autogenous E coli vaccines was administered to pregnant sows in group III for the control. After parturition, antibody titers in colostrum and milk from sows, incidence of diarrhea in suckling piglets, and immunoreactivity in the ileum of piglets from each treatment group were examined. The results were as follows ; 1. Sixty-two strains of E coli were isolated from suckling piglets with diarrhea. Of the strains, K88 pilus and K99 pilus antigens were identified in 6(9.8%) and 4(6.5%), respectively. Molecular weights of K88 and K99 pilus were 27,500 and 18,500 daltons, respectively. 2. Antibody titers in colostrum from sows after parturition were 1 : 512 to 1 : 1,024 in group I, 1.256 to 1.512 in group II, and 1 : 4 to 1 : 16 in group III. 3. The incidences of diarrhea In suckling piglets of group I, II and III were 3.3%, 9.4% and 21.4%, respectively. 4. When the immunoreactivity in the ileum of piglets from each group was examined, the proportion of IgG-immunoreactivity cells in group I or II was higher than that in group III. In conclusion, administration of autogenous E coli vaccines to pregnant sows before parturition can be an effective way to prevent diarrhea in suckling piglets.