Three monoclonal antibody (anti-HN Mab 3115) resistant variants of the Newcastle disease virus (NDV) La Sota strain, were selected (a25, b23, a16); once cloned and shown by haemagglutination inhibition, ELISA and Western blot, not to bind to Mab 3115 they were used as experimental vaccines for chicken. The intracerebral pathogenicity index (ICPI) of a25 and b23 variants was low (0.2 and 0.0 respectively). Three to 4 weeks post-administration of alive variants or inactivated b23, respectively administered via eye drop and subcutaneously, the protection against a challenge was not different from that following La Sota vaccination. Antibody titers induced by a25 and b23, as measured by 2 ELISA blocking tests (the first employing a NDV specific Mab 2114, the second employing Mab 3115), were significantly lower (P less than 0.001) than post-challenge antibody titers. On the contrary, the difference between post-La Sota vaccination antibodies and post challenge antibodies was weak (P less than 0.02). Following 3 successive exposures by contact of chickens to live b23 variant, no variation in antibody titers was observed as measured by ELISA employing Mab 3115. This constituted a necessary criterion, but insufficient to test the stability of the b23 variant. At the same time, the latter exhibited poor ability to diffuse. Vaccination with these variants should be considered in differentiating post-vaccinal from post-infectious antibodies.