Passage of ingested cat immunoglobulin G (IgG) into the hemocoel of cat fleas, Ctenocephalides felis (Bouché), was examined using antibody capture enzyme-linked immunosorbent assays (ELISA) and Western blotting. Fleas were fed heparinized cat blood via membrane feeders. Cat IgG was present in the hemolymph of engorged female fleas 1 h after ingestion at an estimated quantity of 35 +/- 14 micrograms/ml. The prevalence of fleas with demonstrable cat IgG in their hemolymph 1 h after feeding was 100% for both female and male fleas. Following a single blood meal, cat IgG was present in the hemolymph of all 15 fleas tested 1 h after ingestion but dissipated below detectable levels in 10 of 20 fleas examined 3 h after ingestion, and was detectable in only 1 of 10 fleas examined 18 h after ingestion. However, when fleas were provided with continual access to blood over a 72-h period, IgG content in hemolymph, as measured in excised, triturated legs of individual fleas, remained fairly constant (3-16 pg IgG per sample). Flea feeding studies using specific antisera indicated that IgG in flea hemolymph retained its binding activity, and that at least a portion of the IgG was intact. Passage of ingested host antibody from gut into hemocoel is a prerequisite for the possible development of antiflea vaccines that target antigens outside of the flea midgut lumen (e.g., key components of the flea endocrine system controlling oogenesis).