A panel of 10 monoclonal antibodies produced after immunization with two porcine subgroup I rotavirus strains (OSU and A46), and directed against the major inner capsid protein (VP6), fell into six patterns of reactivity when tested against a collection of human and animal group A rotavirus strains. Monoclonal antibodies of pattern I recognized all rotavirus strains. Antibodies of patterns 2 and 3 recognized all subgroup II strains and some, but not all, subgroup I strains. Pattern 4 antibodies identified all subgroup I strains and two strains (H2, equine; CC117, porcine) not reactive with reference subgroup monoclonal antibodies (strains non-I non-II). Pattern 5 antibody exhibited the same reactivity as pattern 4 except for not recognizing the non-I non-II equine strain. Pattern 6 antibodies reacted exclusively with subgroup I and non-I non-II rotaviruses of porcine origin. By competitive binding assays, monoclonal antibodies of patterns 4, 5 and 6 appeared to recognize a single antigenic site, which included at least three overlapping epitopes. In immunoblots all monoclonal antibodies, except one, recognized only the trimeric, but not the monomeric form of VP6.