Three hundred and eleven human sera, obtained from residents in cities of Pelotas, Santiago and Uruguaiana, State of Rio Grande do Sul, Brazil, were tested for haemagglutination inhibition antibodies against reovirus. Antibodies for reovirus type 1 were demonstrated in 58.5%, for reovirus type 2 in 50.5% and for reovirus type 3 in 85.5% of the sera tested.