The potential of 14 different antiscalants (AS) to promote biofouling was determined with the easily assimilable organic carbon (AOC) test and the Biomass Production Potential (BPP) test, respectively. The AOC concentrations of slow sand filtrate supplemented with 50 mg/l of AS ranged from 4 to 112 mug of C/l, and maximum concentrations of adenosinetriphosphate (ATP) from 7 to 380 ng/l, respectively. Concentrations of AOC and BPP were not related to the organic carbon content of the AS's tested. Biofilm Formation Rate (BFR) values of three selected AS's, as determined with the biofilm monitor at a concentration of 2.5 mg/l, were 245, 18.1 and <1.0 pg ATP/cm(2).d, respectively and confirmed the ranking as based on the AOC and BPP tests. Dosage (2.5 mg/l) of one selected AS with a relatively high growth potential caused severe fouling in a 4" RO membrane element supplied with tap water. ATP concentrations on the membrane, including spacers, were 4 times higher in the element supplied with tap water containing AS than in the control. These experimental results agreed with practical experiences, viz. severe biofouling was observed with an AS with high AOC and BPP values and a high BFR value, but no operational problems were observed with an AS with low values for these parameters. Practical experiences also showed that impure mineral acids can cause biofouling. Therefore, selection of chemicals used for scale control on the basis of their growth-promoting properties is needed to limit operational problems. Quality classes based on growth potential tests are proposed for AS's, but further comparison between test results and practice is needed to improve this classification.