The serum of rabbits infected with T. spiralis contained antibodies which on immunodiffusion plates produced 10 or more precipitin bands by reaction with antigens present in whole larval homogenates, and 3 bands with antigens excreted by the larvae into Krebs-Ringer buffer solution. Circular zones of precipitation were also seen near the oral opening of larvae immobilized in a mixture of immune serum and agar. Pre-incubation of larvae or adults for 2 or 18 hr in immune serum did not affect the subsequent absorption of 14C-glucose as compared with larvae pre-incubated in normal serum. Absorption was significantly higher in either normal or immune serum than in KRB-glucose solution. Larvae preincubated in KRB-glucose, normal serum, or immune serum were equally infective to mice, as judged by recovery of adult parasites and secondary larvae. The results did not support the hypothesis that circulating antibodies are of primary importance in immunity to T. spiralis. Present evidence suggests that immunity to T. spiralis during development of the infection is associated with an inflammatory response of the small intestine (Larsh, 1963, 1967a; Denham, 1966). This response may be the result of a specific, delayed hypersensitivity reaction (Larsh, 1967a, b). Humoral antibodies in T. spiralis infections may possibly play a part in immunity by contributing to inflammation through local anaphylaxis, and by interfering with maturation of the larvae to adults. A suggestion made by Sarles and Taliaferro (1936) and Sarles (1938), to the effect that antigen-antibody precipitates forming around migrating larvae of the nematode Nippostrongylus brasiliensis may impair absorption of nutrients may be applicable also to T. spiralis (Oliver-Gonzalez, 1940; Jackson, 1959; Larsh, 1963). Despommier et al. (1967) speculated that such immune precipitates interfered with cuticular absorption of nutrients. An effect of immune precipitates on the uptake of nutrients through the cuticle is unlikely, as there is no reliable evidence for cuticular absorption by nematodes (Castro and Fairbairn, 1969b). On the other hand, an effect of circulating antibodies (immune Received for publication 16 August 1968. * This work was supported by Grant No. AI-04953 and Training Grant 5 TI-AI-00226, from the National Institute of Allergy and Infectious Dis-