A murine model for antigenic competition with multivalent D. nodosus pili vaccine has been established that parallels the phenomenon observed in sheep where levels of antibody, specific for any particular serogroup of pili, are significantly lower following vaccination in the presence of multiple serogroups of pili than with that serogroup alone. This competition was observed in both high and low responder strains of mice and was not dependent on the multiplicity of the antigens in the multivalent vaccine but could be observed with a large excess of a single heterologous serogroup. Competition was manifest by a reduction in the number of serogroup-specific antibody secreting cells elicited in response to vaccination. The antibody response to a single serogroup of pili reached a plateau at high doses and it was at these doses that antigenic competition was most pronounced, under conditions where both B- and T-cell responses were limiting. The limit in T-cell responsiveness was not imposed at the level of presentation of antigen. Pili-specific T cells were largely cross-reactive for different serogroups, and under conditions of limiting T-cell stimulation within a lymph node the available T cells would have to be shared between B cells specific for each serogroup of pili, which may in turn result in the decrease of serogroup-specific antibody induced following inoculation with the multivalent vaccine.