To determine the density of mucosal lymphoid follicles in 10 histologically normal rectums, including the anorectal junction, we counted the follicles macroscopically after the specimens were fixed in 10% acetic acid. The mean density in the rectum was 25.6/cm2. The mean density at the anorectal junction was slightly but not significantly higher, at 29.2/cm2. We found no significant change in follicle density with increasing age in specimens from people between the ages of 17 and 90 years.