Methods are described for: (a) obtaining exsheathing fluid containing leucine aminopeptidase, (b) estimating the activity of the enzyme, and (c) demonstrating the presence of a heat-labile factor which attacks isolated sheaths and which has many of the properties of leucine aminopeptidase. An explanation for the failure of Ozerol and Silverman (1969) to find leucine aminopeptidase in the exsheathing fluid of Haemonchus contortus is given and the evidence for and against the view that this enzyme causes exsheathment is discussed. Rogers (1963, 1965) obtained results which suggested that leucine aminopeptidase might be responsible for the initial breakdown of sheaths during exsheathment. Two species of infective juveniles, Haemonchus contortus and Trichostrongylus colubriformis, were used. Recently Ozerol and Silverman (1969) got evidence from which they concluded "that leucine aminopeptidase is not the enzyme responsible for the exsheathment of H. contortus." This view was based on the findings that (a) the factor which attacked isolated sheaths was heat-stable, and (b) leucine aminopeptidase was absent from their preparations of exsheathing fluid. The reasons why Ozerol and Silverman failed to find leucine aminopeptidase in their preparations of exsheathing fluid are obvious (see Discussion). Their finding that the isolated sheaths may be attacked by a heat-stable substance does not necessarily mean that the system is important in vivo. Indeed their method of preparing isolated sheaths, and the observation in control experiments that Earle's solution alone showed 35% activity, suggest that the sheaths were not normal. Though more than 100 isolated sheaths have been used in experiments described in the present paper spontaneous "ring formation" was never seen and Earle's solution was inactive. It seems, therefore, that the most fruitful way of resolving this controversy would be to provide simple methods which could be widely used, even in class experiments, to show that exsheathing fluid which contains leucine aminopeptidase also contains a heat-labile factor which attacks