When third-stage trichostrongyle larvae exsheathed, they released an exsheathing fluid which eroded the sheath at the anterior end. This led to the formation of a refractile ring and ultimately to the fracture of the sheath which enabled the larva to escape.

Exsheathment was induced by ovine rumen fluid. The exsheathing activity of rumen fluid was impaired by centrifuging, filtering, boiling and aeration, but activity could be restored by the addition of reducing agent. The activity in vitro of either fresh rumen fluid or freeze-dried rumen fluid, after addition of a reducing agent, was comparable with that obtained in vivo.

After stimulation, the process of exsheathment continued in the absence of rumen fluid. The ability of rumen fluid to stimulate larvae was dependent upon temperature, hydrogen-ion concentration and oxidation-reduction potential.

Exsheathing fluid was present in larvae both before and after exsheathment. Larvae which had been stimulated to exsheath released exsheathing fluid for a brief interval and further release could be obtained only after the stimuli had been temporarily withdrawn.

The mechanisms for reception of the stimulus to exsheath and for storing and releasing exsheathing fluid were located in a region between the base of the oesophagus and the excretory pore. The fluid was probably released from the excretory pore.

Exsheathing fluid from one species was not necessarily as active against sheaths from other species as against its own. The activity of exsheathing fluid was dependent upon the presence of a heat-stable cofactor of low molecular weight. Activity of the dialysed fluid could be restored by addition of MnCl2 or MgCl2. Values of Q10 for the action of the fluid on the sheath lay between 1·3 and 1·9. Activity was inhibited by Hg2+, Fe3+ and iodoacetic acid.