The mechanisms involved in the induction of the immune response in humans or experimental hosts infected with Giardia intestinalis are not well understood. The results of previous studies indicate that the parasite induces a mixed Th1/Th2 response and that, in experimentally infected mice, the parasite's excreted/secreted (E/S) proteins contain cysteine proteases that are recognised by the murine immune system. In the present study, the possible effects of the E/S proteases of G. intestinalis on the host's humoral and cellular immune responses were investigated in BALB/c mice immunized with the parasite's E/S proteins. High titres of specific IgG(1), IgG(2a) and IgE antibodies were detected after immunization with native E/S proteins. Spleen cells stimulated with such proteins in vitro showed a significant antigen-specific proliferative response accompanied by the production of high concentrations of interleukin-4 (IL-4), interleukin-5 (IL-5) and interleukin-10 (IL-10) but little secretion of interferon-gamma (IFN-gamma). When, before use, the proteases in the E/S proteins were inhibited, by heat treatment or the addition of E-64, they elicited much lower titres of specific IgG(1) and IgE in mice while, in splenocytes in vitro, they triggered much lower production of IL-4, IL-5 and IL-10 and reduced antigen-specific proliferation. Since E-64 only inhibits cysteine proteases, these results indicate that the excreted/secreted cysteine proteases of G. intestinalis may be involved in the induction and regulation of a specific immune response in the infected host.