Monochloramine is becoming a popular alternative to chlorine for the disinfection of water supplies because it does not react with humic substances to form trihalomethanes. However, chloramines have recently been found to cause hemolytic anemia in patients undergoing kidney dialysis. A study of Urbana-Champaign, Ill., tap water was carried out to determine the effects of particle size, contact time, and dissolved organic matter on carbon column effluent concentration when a typical influent monochloramine concentration was used. The resulting data allow a good estimate to be made of the best design for a carbon column, but until more data become available, dechlorination units that are being used for the removal of monochloramines should be carefully monitored to ensure that water of the desired quality is being produced.