The buffering capacity was determined in the extracellular aqueous phase of 24-h plaque samples obtained from dental students. The mean pH of 5 control samples of plaque fluid was 6.54 .+-. 0.26 SD. In the range pH 6-5, the mean buffering capacity was 3.2 .times. 10-4 mol .DELTA.aH+[change in H+ activity]/mol H+ added while in the range pH 5-4 it was 1.8 .times. 10-3 mol .DELTA.aH+/mol H+ added, indicating a reduction in buffering at the lower pH range. Buffering between pH 5 and pH 4 was essentially reversible, while between pH 6 and pH 5, it was associated with the irreversible loss of a component which precipitated out. Buffering above pH was also associated with the irreversible loss of a component. At least 3 buffer components occur in plaque fluid and rinsing with deionized water reduced the buffering capacity in plaque field.