The exposure of pigs to different pathogens compromises productivity (Greiner et al., 2000), i.e. reduces weight gain and food intake, even in the absence of clinical disease. Associations between productivity and a range of immunological traits have been demonstrated in apparently healthy animals facing the same infectious challenge (Clapperton et al., 2003). Specifically, it was demonstrated that some leucocyte proportions (monocytes, B cells, MIL-4 (NK) positive cells) increased significantly as performance decreased. This study aims to verify these previous results on an independent population of pigs, examine relationships between immune measures and actual bacterial counts in the gut, and examine the influence of diet type on these measurements.