Abdomen homogenates of refractory C. quinquefasciatus and Anopheles stephensi prepared from equal and varying wet weights of abdomen, were shown to inhibit endopeptidase induced in vitro exsheathment of Brugia pahangi microfilariae. This inhibitory effect was removed by heat treatment and centrifugation of the homogenates. Abdomen homogenate from susceptible Aedes aegypti did not inhibit exsheathment. This suggested the presence of some inhibitory factor in the abdomens of the two insusceptible species of C. quinquefasciatus and An. stephensi. There was no evidence of this inhibitory effect in the midgut homogenates of the three mosquito species. Foetal calf serum and bovine serum albumin also inhibited endopeptidase induced exsheathment of B. pahangi microfllariae, suggesting that the protein concentration of the proteinaceous substances also determined the extent of inhibition. In the absence of endopeptidase, incubation in midgut homogenate of the 3 species of mosquito did not initiate exsheathment, suggesting that the conditions present in vitro did not simulate those persisting in the mosquito midgut in vivo.