Of the more than 450 cattle over 24 months of age in ten Johne's infected herds cultured at six month intervals, 106 were positive on two or more fecal culture tests. The pattern of detection by fecal culture using a centrifugation technique for each of the ten herds varied enormously. Each of the herd owners were encouraged to remove known infected animals but animals with the higher colony counts were judged the greatest risk to spread the infection and generally culled first. Occasionally animals with only a few colony counts would not be detected again by culture for two years while other animals had a more consistent increase in their colony counts over time. Some animals were detected by only one culture with often less than five colonies total on the four tubes. These animals were not detected again until followed to slaughter and tissues cultured for M. paratuberculosis. Nearly always each of these animals with only a few colonies had numerous colonies isolated from a variety of tissues. Approximately 50% of the animals determined to be infected were detected on the first herd culture and the remainder were detected on subsequent herd cultures over the four year period. Thus, sensitive fecal culture techniques were able to identify only half of the infected animals in a herd at one point in time. Unfortunately most of the infected animals not detected by culture were also negative by most serologic tests performed on a subset of the population.