A method is described for the isolation of pure, chemically intact sheaths of blood microfilariae of Litomosoides carinii. Microfilariae were isolated according to standard techniques. Exsheathment was performed by freezing-thawing-shaking procedures, repeated 5-10 times, i.e., larvae were frozen in liquid nitrogen, thawed at room temperature, and shaken vigorously for 5 s. Exsheathment rates were about 50%. Sheaths were separated from ensheathed and exsheathed microfilariae and microfilariae fragments by filtration through a polycarbonate filter (2 micron pore size). The achievable yield (about 15% of the sheaths of a batch of microfilariae) was approximately 1 microgram of sheaths per 10(6) microfilariae.