The purposes of this study were (a) to measure serially ascitic fluid polymorphonuclear cell response in treated spontaneous bacterial peritonitis and (b) to determine whether an ascitic fluid polymorphonuclear cell count of less than 250 per mm3 on serial paracenteses was a satisfactory endpoint for antibiotic therapy. Thirty of 33 patients showed an exponential fall in ascitic fluid polymorphonuclear cell count after 48 hr of antibiotic therapy; the magnitude of decrease correlated with survival (p less than 0.01). Among the patients whose antibiotic therapy was discontinued when the ascitic fluid polymorphonuclear cell count reached 250 per mm3 or less, the duration of therapy was considerably shorter than for the patients who received "conventional" therapy (p less than 0.01). Recurrence of spontaneous bacterial peritonitis was similar in the two groups. Mortality correlated with the severity of underlying liver disease but not with duration of antibiotic therapy.