Our lab is currently interested in the mechanisms responsible for the transition of Johne's disease from a clinically latent infection to clinical disease marked by progressive bacterial multiplication, fecal shedding, chronic diarrhea and wasting. These events have been reported to be associated with parturition and onset of lactation. We developed a hypothesis that the onset of clinical disease may be associated with changes in serum hormone levels due to hormone cycling at parturition. We, therefore, tested several of the major hormones that cycle at this time. Of the hormones tested, we found that two can have a profound effect on intracellular growth of M. paratuberculosis in primary bovine monocyte cell cultures. These were bovine growth hormone (GH) and bovine prolactin. In both cases, bacillary multiplication increased to near maximal doubling times when infected monocyte cultures were exposed to hormone for the first 5 days of a 12 day incubation. GH and prolactin treated cultures at day 12 showed a 1.5 to 2 Log10 increase over controls that were not treated with hormone. Furthermore, our data indicated that phagocytosis of M. paratuberculosis was decreased in a dose dependent manner at GH concentrations from 1 to 10 ng/ml, and nearly eliminated at a concentration of 50 ng/ml.