Using enzyme linked immunosorbant assays (ELISA) we compared the suitability of liver fluke homogenate (LFH), excretory/secretory products (ES) and cathepsin LI as a diagnostic agent for the serodiagnosis of human fasciolosis in the Bolivian Altiplano. Cathepsin LI (CL1) proved significantly better at the discriminating between seropositive and seronegative Fasciola hepatica infected individuals when compared to the other antigen preparations. The sensitivity of the CL1 -ELISA was further improved by using secondary antibodies directed against IgG4 only. This IgG4/CLl-ELISA was highly specific because sera obtained from individuals infected with other parasites were negative in the assay. We also employed recombinant CL1 as antigen in this ELISA. A statistically significant correlation (r2 = 0.751; p<0.001) was observed between recombinant and native proteins making the standardisation of this assay possible. The difficulty of obtaining blood from the indigenous population prompted us to explore the method of collecting samples of blood onto filter paper after lancet-pricking of the finger (blood-filter samples). Statistical analysis revealed that there was significant correlation between the results obtained with serum and blood-filter samples (r2= 0.848; p<0.001). The collective serological data revealed that 44.3% of the 360 individuals in the Bolivian Altiplano tested were diagnosed as serologically positive. In addition, our data shows that fasciolosis has been endemic in this region for at least a decade. Fasciola hepatica infection in mice was associated with a type II immune response which results in a generalised Th-subset imbalance polarising towards a Th2 cytokine profile. An established type I immune response in mice vaccinated with B. pertussis whole cell vaccine was significantly down-regulated, in an IL-4 dependent process, when these mice were subsequently infected with F. hepatica. We also demonstrated that intravenously administered F. hepatica ES products could induce the down-regulation of the type I immune response in these vaccinated mice. In addition, mice injected intraperitoneally with ES exhibit a predominant Th2 cytokine profile. Peritoneal exudate cells secrete IL-10 in vitro when stimulated with ES suggesting that this cytokine may be the involved in the initial stages of inducing a type II response.