The AA. studied the reaction of indirect haemoagglutination for the determination of HBsAg in 157 sera from patients with viral hepatitis, with 3 differents commercial tests based on the agglutination by HBsAg, respectively, of sheep, turkey and human erythrocytes sensitized with antibody to HBsAg. All three techniques showed a high sensibility comparable to that of the RIA. Regarding specificity the best results were obtained with the turkey and human erythrocytes, while the sheep erythrocytes because of the presence of some aspecific positivity, need a confirmatory test.