Immune sera have previously been shown to play a positive role in immune protection against murine experimental brucellosis. The protective properties of a panel of five anti-Brucella monoclonal antibodies (Mabs) were therefore assessed by estimation of the acceleration of the blood clearance of intravenously inoculated Brucella and of the reduction of splenic infection on Day 7 after infection. Three 'strongly protective', one 'weakly protective' and one 'non-protective' Mabs were identified. As a first step towards the study of the mechanism of this humoral protection, these Mabs were further compared for structural and functional properties such as immunoglobulin isotype, anti-Brucella specificity, anti-Brucella in vitro bacteriostasis, Brucella agglutination and complement fixation when complexed with tyndallized Brucella. No correlation was found between protection and either agglutination or direct bacteriostasis. On the other hand, the results observed suggest that isotypes (and especially the IgG2a isotype) could play an important role in in vivo immuno protection and that complement may be involved. However, the fact that one of the protective Mabs belongs to the IgA isotype, does not cross-react with the others in anti-Brucella epitopic specificity and does not fix complement underlines the probable diversity of the mechanisms involved.