Antibody responses were measured in a volunteer infected four times with Necator americanus over a 27-month period. The main source of antigen was culture fluid in which living adult N. americanus had been maintained for several days. Antibodies to worm acetylcholinesterase and IgE antibodies were detected only with this material, but antibodies were identified by the enzyme-linked immunosorbent (ELISA) assay, with either adult worm secretions or extracts of third-stage infective larvae. The total serum IgE level fell after the first infection, but although it then increased during subsequent infections, it never rose above 600 U per ml. None of the antibody responses suppressed the rat of worm development to maturity, or reduced the fecundity of the parasites. However, it is suggested that the development of the immune response may be associated with the waning of the severe gastro-intestinal symptoms which were experienced in this infection, and which are frequently characteristic of hookworm infections.