This article evaluates the role of alpha-fetoprotein (AFP) testing in the diagnosis of ruptured membranes in the preterm patient. AFP is normally found in amniotic fluid and is not present in vaginal secretions. Testing for AFP in vaginal fluid was compared with nitrazine evaluation and ferning in two groups of preterm patients, those with intact membranes and those with ruptured membranes. There was no difference in the sensitivities of AFP, nitrazine, or ferning in diagnosing ruptured membranes. The specificities of AFP and ferning were similar and both were significantly better than nitrazine. Duration of membrane rupture for greater than 24 hours negatively affected the accuracy of all three tests. Blood-tinged amniotic fluid interfered with AFP and nitrazine evaluation. AFP does not appear to offer a significant advantage over nitrazine testing and ferning in the evaluation of patients with possible ruptured membranes.