The indirect conclusion that the sheath of the microfilariae (mf) of Brugia malayi, as well as in other species, is mainly responsible for the protection against the host immune response is well described (1979, Tropical Diseases Research Series 2, W.H.O.). However, research on the nature of the variant specific surface antigens on the sheath has been hindered by the lack of an appropriate method for its isolation in a pure form. An advance in circumventing this problem has been the development of a method of exsheathment of mf of B. malayi and the collection of the sheath in a reasonably pure form. In the present study, B. malayi mf were obtained from the blood of heavily infected Mastomys natalensis by the method of Obeck (1973, The Journal of Parasitology, 59: 220-222). The blood was placed on a 150 x 15 mm plastic petri dish, covering 1/3 of its area, and allowed to clot at least 4 hr at 25 C. The petri dish was then tipped slightly to allow the serum containing mf to collect at one end of the dish. After 20 hr, the clear pool of serum was collected and mf were concentrated by centrifuging at 250 g for 10 min. The pellet obtained was then washed 3 times and finally the mfwere suspended in Ca2+ free Hank's balanced salt solution (HBSS). Studies on exsheathment involved the incubation of 106 mf in Ca2+ free HBSS with one of the following proteolytic enzymes: endopeptidase, papain, trypsin, chymotrypsin, peptidase, collagenase, and protease in a total volume of 5.0 ml at 37 C. An aliquot of 10.0 ,l was removed from the tube at 4 hr intervals from the start of incubation and continued up to 24 hr; the mf were examined for exsheathment using a light microscope after staining wih Giemsa. Results showing the percentage of exsheathment that occurred during incubation with different proteolytic enzymes are summarized in Table I. The concentration of enzymes in the medium yielded maximum exsheathment under the specified conditions and all the mf were viable during the entire period of incubation. Ninety-two percent exsheathment of the mf occurred within 8 hr in solutions containing endopeptidase. Eighty-eight percent of the mf exsheathed within 4 hr in the presence of papain. Collagenase and protease in the medium also induced exsheathment but at a low rate, as about 54 and 16% exsheathment respectively took place at the end of 24 hr. The enzymes trypsin, chymotrypsin, and peptidase failed to stimulate any exsheathment, and confirming the observations of Devaney and Howells (1979, American Journal f Tropical Medicine and Hygiene, 14: 227-233) on their studies with mf of B. pahangi. Separation of the casted sheath from the incubation medium was tried using the technique of sucrose density gradient centrifugation, based on centrifugal or gravitational flotation in a solution intermediate in the specific gravity between the sheath and the intact or exsheathed mf. The following sucrose solutions were used to prepare the gradients: 3% (specific gravity 1.0099), 13% (specific gravity 1.0507), 24% (specific gravity 1.0990), 35% (specific gravity 1.1513), and 54% (specific gravity 1.2518). Two ml of each solution was carefully layered by hand pipetting in turn from the heaviest solution (54%) to the lightest (3%) in a 15 ml conical graduated centrifuge tube. The tube was then stoppered and