Plate count agar is presently the recommended medium for the standard bacterial plate count (35C, 48 h incubation) of water and wastewater. However, plate count agar does not permit the growth of many bacteria that may be present in treated potable water supplies. A new medium was developed for use in heterotrophic plate count analyses and for subculture of bacteria isolated from potable water samples. The new medium, designated R2A, contains 0.5 g yeast extract, 0.5 g Difco Proteose Peptone No. 3, 0.5 g Difco Casamino Acid, 0.5 g glucose, 0.5 g soluble starch, 0.3 g K2HPO4, 0.05 g Mg SO4.7H2O, 0.3 g sodium pyruvate, and 15 g agar per liter of laboratory quality water. Adjust pH to 7.2 with crystalline K2HPO4 or KH2PO4 and sterilize at 121C for 15 min. Results from parallel studies with spread, membrane filter and pour plate procedures showed that R2A medium yielded significantly higher bacterial counts than did plate count agar. Studies of the effecton temperature.