Fifty-six patients with alcoholic cirrhosis and ascites were studied. The ascitic fluid was analyzed for pH, PO PCO2, glucose, protein, specific gravity, amylase, lactic dehydrogenase, white blood cell count, polymorphonuclear count, and cytology. It was also cultured aerobically and anaerobi-cally. Simultaneously, arterial blood was analyzed for pH, PO2, and PCO2. Venous blood was analyzed for complete blood count, protein, aspartate transaminase, and it was also cultured under aerobic and anaerobic conditions. Six patients had spontaneous bacterial peritonitis (SBP), i.e., culture was positive for Escherichia coli in five and Streptococcus faecalis in one. The mean (±S.E.) ascitic fluid pH in the SBP group was 7.25 ± 0.06 with a range of 7.12 to 7.31, while the ascitic fluid pH in the group with sterile ascites was 7.47 ± 0.07 with a range of 7.39 to 7.58. The pH of the blood in both groups was 7.47 ± 0.03. The pH of the ascites in the SBP group was significantly different from the pH in the group with sterile ascites, p < 0.001. It was also significantly different from the blood pH, p < 0.001. Highly significant inverse correlations existed between the ascitic pH in the SBP group and the ascitic white blood cell count (r = –0.84, p < 0.01) and between the ascitic pH in the SBP group and the ascitic polymorphonuclear count (r = −0.87, p < 0.01). The ascitic fluid pH is recommended as an easy, quick, sensitive, and specific means of diagnosing SBP and it overcomes the problem of the present SBP diagnostic methods of utilizing an absolute white blood cell count > 500 per mm3 or a polymorphonuclear count > 250 per mm3 in which false positive interpretations occur.