To study the efficacy of colon cleansing regimens, 500 hospital outpatients were randomly assigned to one of four regimens. Two featured castor oil followed by phosphate enema and two used magnesium citrate and bisacodyl. For the 349 patients who completed all components of the regimens, quality of the subsequent barium enema examination was scored by three radiologists in a blind manner. Regimens featuring magnesium citrate and bisacodyl were found to be significantly superior to those featuring castor oil. Calcium bisdiocytl sulfosuccinate did not improve the quality of colon cleansing but appeared to decrease the incidence of abdominal cramping in patients receiving magnesium citrate and bisacodyl. It is hoped that the investigative approach used in this study may serve as a model for future testing of colon cleansing regimens.