The potentiation effect of various adjuvants on the production of guinea-pig IgE was investigated using Freund's complete and incomplete adjuvant, the lipopolysaccharide of Escherichia coli and Salmonella typhosa, Bordetella pertussis, and the nematodes Nippostrongylus brasiliensis and Ascaris suum. While all the antigens had a variable effect on the potential of the IgG response, only infection with A. suum resulted in an enhanced IgE response to the antigen, egg albumin. Maximum potentiation occurred when primary immunization and nematode infection were accomplished simultaneously.