Avian Metapneumovirus (aMPV), also called Turkey Rhinotracheitis Virus (TRTV), is an upper respiratory tract infection of turkeys, chickens and other avian species. Five monoclonal antibodies (MAbs) were created against the Brazilian isolate (SHS-BR-121) of aMPV, MAbs 1A5B8; 1C1C4; 2C2E9 and 2A4C3 of IgG1 and MAb 1C1F8 of IgG2a. Four Mabs (1A5B8; 1C1C4; 2C2E9 and 2A4C3) showed neutralizing activity and three (1A5B8; 1C1C4 and 2A4C3) inhibited cellular fusion in vitro. These MAbs were used to investigate antigenic relationship among three strains (SHS-BR-121, STG 854/88 and TRT 1439/91) of aMPV subtypes A and B using cross-neutralization test. The results confirm that the monoclonal antibodies described can be used as a valuable tool in the epizootiological and serological studies, and also for the specific diagnosis of the subtypes in the infection for Avian Metapneumovirus.255258Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)