Screening of vaccines against intracellular pathogens in ruminants is often difficult, costly and time consuming. These three factors limit the number of candidates that can be screened which in turn can decrease the chances of finding a favourable vaccine. In vitro cellular infection assays can function as an alternative preliminary vaccine screening tool, and also complement and extend findings derived from lager ruminant vaccine trials. With the aim of eliminating some of the above factors that make vaccine screening difficult, we have developed a rapid assay that can be utilised to screen a large number of vaccine candidates. The in vitro assay examines the ability of peripheral blood mononuclear cells (PBMCs) from vaccinated animals to kill intracellular bacteria. In brief, PBMCs are separated via adherence, and the adherent monocytes are infected with Mycobacterium avium subspecies paratuberculosis (MAP), an intracellular ruminant pathogen. Non-adherent PBMCs are returned to the well after infection and co-cultured. The quantity of viable intracellular MAP is then determined using a novel method based on the relationship between culture growth rate and concentration of live MAP at the start of culture. Results of the in vitro assay show a large variation in the killing ability of PBMCs from non-vaccinated control animals. In contrast vaccination decreases the variability between animals and increases in vitro killing of MAP. Due to the small number of vaccinated sheep that developed clinical disease in this trial it was not possible to differentiate the responses of diseased and non-diseased vaccinates. However the assay has proven to be useful for examining the host response to vaccination and clearly distinguishes vaccinates from unexposed animals.