Third and artificially hatched second-stage larvae of A. suum were exposed to the action of saline and normal and homologous immune pig serum. Precipitates developed at the mouth, excretory pore, anus and around the cuticle of third-stage larvae when they were placed in immune pig serum, but not in saline or normal serum. Precipitate was most evident at the excretory pore and was often the first precipitate to form. It is suggested that the excretory products may play an important role in stimulating the production of antibody in the host. A relationship was noted between the antibody titre of the serum and the amount of precipitate, the life span of the larvae, their activity, and the rate at which precipitates formed. It is concluded that the titre of the serum was in some degree a measure of the ability of the host to resist reinfection.

Circumlarval precipitates did not form when second-stage larvae were placed in samples of the same immune serum. It is pointed out that this may be due to the lack of sufficient antigenic material available to react with antibody in the antisera. It is also suggested that an absence of sufficient larval antigen may also partly account for the small degree of resistance exhibited by the resistant host within the first 2 or 3 days of reinfection.

The work, which is abstracted from a Ph.D. thesis, was carried out in 1957 under the tenure of an Animal Health Trust Wellcome Fellowship. I am grateful to Dr Soulsby, under whose guidance this work was carried out.