Studies were carried out on cysts mechanically isolated from golden hamsters after 50 to 60 days of infection with T. spiralis. It was found that the outer hyaline wall of the cyst was digested by clostridial collagenase prepared as a 1% solution in phosphate buffer (pH 7.6; 37 C). The time required for digestion (15 to 25 min) was similar to that observed with pepsin. Powdered hide collagen and fresh hamster Achilles tendon were also dissolved under the same conditions; muscle proteins were resistant to the crude collagenase. Heat-denatured cysts were more slowly digested than the native structure. Buffer controls and heat-inactivated enzyme showed no activity against cysts. Trypsin, pan- creatin, and hyaluronidase had no significant action on native cyst walls during a 6-hr period of incu- bation. Trypsin was effective in destroying the cyst wall after 18 to 24 hr of exposure at 37 C. Trypsin was rapidly effective (35 min) in digesting the hyaline walls of heat-denatured cysts. The outer cyst wall of trichinella was inadequately formed in guinea pigs on a scorbutic diet. Infected guinea pigs which were pair-fed with the same diet were able to form normal cyst walls when they received daily supplements of ascorbic acid or orange juice. The walls of isolated whole cysts were birefringent to polarized light. Native cyst walls and tendon collagens from the same host shrank at 62 C which is the thermal shrinkage temperature of mammalian collagen. Cyst walls were resistant to mineral acid and urea; they gave positive reactions for protein with several standard spot test reagents. A 1% solution of sodium hypochlorite caused the immediate dissolution of the outer walls of whole cysts. Reaction with "collagen" stains, sensitivity to dietary ascorbic acid, resistance to trypsin, and collagenase susceptibility are regarded as evidence that the principal structural element of the cyst wall of trich- inella is collagen.