We studied the relationship of amniotic fluid bacterial inhibition in 50 patients with intra-amniotic infection to that in 50 matched control patients. Amniotic fluid was collected through a transcervical intrauterine catheter. All infected patients had clinical signs of intra-amniotic infection and greater than or equal to 10 2 colony-forming units per milliliter of a high-virulence organism. None of the control patients became infected. The matching characteristics for the intra-amniotic infection group versus the control group were: interval from rupture of the membranes to delivery (14.30 +/- 7.96 versus 15.00 +/- 7.19 hours, NS), interval from rupture of the membranes to collection of amniotic fluid (12.41 +/- 6.37 versus 12.16 +/- 6.46 hours NS), and gestational age (40.3 +/- 1.6 versus 40.0 +/- 1.6 weeks, NS). All patients were in labor. We tested each sample of amniotic fluid for inhibitory activity to Escherichia coli by a plate-count technique. Thirty-five samples (70%) of intra-amniotic infection fluid were noninhibitory, whereas 16 samples (32%) of control fluid were noninhibitory. The conclusion was that amniotic fluid from patients with intra-amniotic infection was significantly less likely to be inhibitory to E. coli (p less than 0.001).