A peculiar pattern of antibody response to human coronavirus (HCV) OC43 and neonatal calf diarrhea coronavirus (NCDCV) was observed in a significantly higher proportion of children with gastroenteritis (GE) than controls. It was directed only to OC43 surface antigens and appeared to be quite different form that observed in OC43 respiratory infections, in which an antibody response to both OC43 and NCDCV surface and internal antigens was found. Subsequently, fecal excretion of CV-like particles was detected by electron microscopy in a significantly higher proportion of infants and young children with acute GE as compared to controls. Immune electron microscopy showed a two-way cross-reactivity between HCV OC43 and human enteric coronavirus (HECV), both using immune sera (raised in animals immunized with HECV strains purified from stools of two patients) and patients' convalescent sera. These data show that HECVs are antigenically related to HCV OC43 and could be somewhat involved in the etiology of GE in infants and young children.