Breakdown of a native chlamydial vaccine against ovine enzootic abortion (OEA), coupled with the identification of the protective capacity of the major outer membrane protein (MOMP) of C. psittaci, has encouraged research into the use of recombinant forms of MOMP in a third generation OEA vaccine. The prospect of testing all recombinant forms of MOMP in pregnant sheep trials is a daunting one, both in terms of time and money. To circumvent these problems, it was decided that a model system would be required to assess the efficacies of recombinant MOMP constructs. The initial model developed (active immunisation of mice) did not appear to reflect immunisation in sheep. Difficulties were encountered in eliciting seroconversion in mice vaccinated with recombinant antigens. It also demonstrated a possible H-2 link to the MOMP-specific antibody response. Problems associated with the model were overcome to some extent, by the development of a second model (passive transfer of sheep sera to mice). In this case, it was demonstrated that infection of mice susceptible to C. psittaci, could be reduced by passively transferring sera from sheep which had been vaccinated with recombinant antigens. The mechanism of neutralisation in vivo was also examined for this model. The model was further used to identify protective regions within the MOMP by passive transfer of affinity purified antibodies and monoclonal antibodies. Significant protection was afforded by the VS1 and VS2 regions of the MOMP.