Manure slurries (n = 3) prepared from the feces and urine of lactating dairy cattle (1 part urine, 2.2 parts feces, and 6.8 parts distilled water) had an initial pH of 8.6 +/- 0.1; dissolved carbonate concentrations of 48 +/- 4 mM, and Escherichia coli counts of 5.9 +/-0.7 logs per mi slurry. The pH of untreated slurries declined to pH 7.0 +/- 0.1 by the 10th day of incubation, and the E. coli count increased approximately 10-fold (P < 0.05). When slurries were treated with Na2CO3, K2CO3, NaHCO3 or Na2CO3NaHCO3 (0 to 16 g/kg slurry), the dissolved carbonates increased in a linear fashion, but only Na2CO3 and K2CO3 (8 g/kg or greater) or Na2CO3. NaHCO3 (16 g/kg) ensured an alkaline pH. Even relatively low concentrations of Na2CO3 of K2CO3 (8 or 12 g/kg) caused a decrease in E. coli viability (P ( 0.05), and E. coli could not be detected if 16 g/kg was added (day 5 or 10 of incubation). Na2CO3. NaHCO3 also caused a decrease in E, coli viability, (P < 0.05), but some E. coli (approximately 10(4) cells per g) were detected on day 10 even if the concentration was 16 g/kg. NaHCO3 did not prevent the decrease in pH or cause a decrease in E. coli numbers (P > 0.05). Calculations based on the Henderson-Hasselbalch equation (pH and dissolved carbonates) indicated that little E. coli killing was noted until the dissolved carbonate anion concentrations (CO3-2) were greater than 1 mM, but bicarbonate anion (HCO3-) concentrations as high as 180 mM did not affect E. coli viability. These results are consistent with the idea that carbonate anion has antimicrobial properties and can kill E. coli in dairy cattle manure.