This report describes the effect on various stages in the life cycle of the nematodes T. spiralis and N. brasiliensis of complement, antibodies from rats infected with these parasites and several different cell types. The cuticle of the infective larvae and adult worms of both nematode species activates complement via the alternative pathway, but the cuticle of newborn T. spiralis lacks this property initially. As newborn larvae grow, however, the newly formed cuticle in the midregion of their body is able to activate complement. Rats infected with either nematode species produce antibodies to the cuticle of all life cycle stages which show marked specificity to each stage in the life cycle. Whereas the cuticle of T. spiralis reacts evenly over the entire surface both to complement and to antibodies, the reaction of the cuticle of N . brasiliensis to either reagent is patchy. Infective larvae of N. brasiliensis were killed in vitro in the presence of complement, by neutral red-positive peritoneal macrophages which were nonadherent to plastic. The infective and newborn larvae of T. spiralis were killed by eosinophil-enriched cell populations and antibodies. The speed of eosinophil killing of the T. spiralis larvae was enhanced when the serum was freshly collected and when the eosinophil suspension also contained neutral red-positive nonadherent macrophages. Newborn larvae of T. spiralis and infective larvae of N . brasiliensis assumed a rigid appearance at death. Infective larvae of T. spiralis burst, extruding their internal organs through their cuticle weakened by antibodies and the cells.