Infections with parasitic helminths such as Nippostronglyus brasiliensis induce dominant type 2 responses from antigen‐specific T helper cells. The potency of the Th2 bias can also drive Th2 responses to bystander antigens introduced at the same time as infection. We now report that the Th2‐promoting effect of infection can be reproduced with soluble N. brasiliensis excretory‐secretory proteins (NES) released by adult parasites in vitro. Immunization of BALB / c mice with NES results in the production of IL‐4 with elevated total serum IgE and specific IgG1 antibodies. NES is also able to stimulate IL‐4 and polyclonal IgE production in other mouse strains (C57BL / 6, B10.D2, CBA). These features are seen whether NES is administered without adjuvant as soluble protein in phosphate‐buffered saline or with complete Freund's adjuvant which normally favors Th1 responses. Thus, NES possesses intrinsic adjuvanticity. Moreover, co‐administration of hen egg lysozyme (HEL) with NES in the absence of other adjuvants results in generation of HEL‐specific lymphocyte proliferation, IL‐4 release and IgG1 antibody responses, documenting that NES can act as an adjuvant for third‐party antigens. Proteinase K digestion or heat treatment of NES before immunization abolished the IL‐4‐stimulating activity, indicating that the factors acting to promote Th2 induction are proteins secreted by the adult parasite.