Ascitic fluid pH and lactate concentration have been proposed as useful tests for the detection of ascitic fluid infection. However, past studies involved small numbers of infected patients, and all did not use optimal culture techniques. This large study was performed using highly sensitive culture methods and sought (a) to compare the sensitivity, specificity and accuracy of pH and lactate to that of the ascitic fluid neutrophil count and (b) to determine whether evaluation of ascitic fluid pH or lactate (or arterial-ascitic fluid pH or lactate gradient) would result in improved decision-making regarding empirical treatment of suspected ascitic fluid infection. Analysis of 206 ascitic fluid specimens obtained in 175 patients, including 101 infected specimens, revealed that ascitic fluid (or arterial-ascitic fluid) pH and lactate were less than 50% sensitive in detecting bacterial peritonitis and that these tests did not improve clinical decision-making about empirical treatment of suspected ascitic fluid infection. Although statistically significant differences in ascitic fluid pH were detected between infected samples and control samples, these differences did not appear to be clinically helpful. The ascitic fluid pH was 0% sensitive in detecting the presence of bacteria in the absence of neutrophils (i.e., no such specimens had a pH lower than 7.35). Ascitic fluid pH correlated well with neutrophil count and appears to be, at least in part, an indirect measure of the presence of neutrophils in ascitic fluid. Measurement of pH or lactate (or arterial-ascitic fluid gradients) is not helpful in the clinical management of infected ascitic fluid.