The competitive exclusion (CE) action of Aviguard (AG) and its effects on the antibody response of chicks were evaluated in this study. We observed that AG protected the chicks from overwhelming colonization. Fourteen days after infection, fewer AG-pretreated than nonpretreated chicks shed salmonellae from their coloaca in both infected groups, although much less from SE-infected chicks. Antibody titers of sera produced to Salmonella typhimurium (ST) and SE in pretreated and non-pretreated chicks were not significantly different. Immunoblotting showed that these antibodies reacted with SDS-PAGE-separated 71.4, 67.7, 44.0, and 30.3 kDa proteins detectable in the test strains. Few weak bands of doubtful significance were observed in the cross-reaction between the sera of ST- and SE-infected chicks with ST and SE antigens, respectively. Our study showed that AG protected chicks from overwhelming colonization by salmonellae, and neither altered the antigenic proteins of infecting salmonellae nor their recognition by specific antibodies produced in response to the infection.