Data are presented on the sensitivity, specificity, and positive predictivity of the Hemoccult test based on the experience of the Minnesota Colon Cancer Control Study, a randomized clinical trial to determine whether the use of the Hemoccult test can reduce mortality from colorectal cancer. Rehydrating the slides with a drop of water before processing resulted in an increase in positivity (2.4% to 9.8%), and sensitivity (80.8% to 92.2%) but a decrease in specificity (97.7% to 90.4%) and positive predictivity (5.6% to 2.2%). The effects of age and sex were also evaluated. The test was less specific for men than women (p = 0.03). Specificity was highest for those < 60 yr of age and decreased with increasing age (p = 0.05). The positive predictivity increased with age from 1.6% for those under 60 yr to 3.6% for those over 70 yr (p = 0.0004).