Seventy-five paired ascitic fluid and serum specimens were tested for C-reactive protein (CRP) concentrations with the use of laser nephelometry. There was no clear separation of the ascitic fluid or serum values of the 19 paired specimens obtained from patients with peritonitis from the 37 paired sterile portal-hypertension-related samples or from the 19 paired miscellaneous specimens. The ascitic fluid CRP concentrations of patients with sterile portal-hypertension-related ascites were not significantly different from those of infected specimens. However, the serum CRP values were significantly higher in patients with peritonitis than in patients with sterile portal-hypertension-related ascites. Ascitic fluid CRP does not appear to be a useful indicator of ascitic fluid infection.