We have developed an ELISA for the diagnosis of human fascioliasis based on the detection of IgG4 antibodies to Fasciola hepatica cathepsin LI cysteine protease. Use of this assay in the Bolivian Altiplano, a region with a high prevalence of the disease, was hampered by the reluctance of the indigenous population to provide blood. To overcome this problem, we have investigated the method of collecting small quantities of blood from the finger onto filter paper, followed by the elution of antibodies for use in the diagnostic assay. Serum samples and blood samples collected onto filter paper were obtained from 57 individuals living in the village of Cutusuma in 1987 and from 11 individuals in Chijipata in 1996. Analysis of the IgG4-ELISA results revealed that there is highly significant linear relationship (P < 0.001) between the two methods of sampling. Most importantly, a reliable diagnosis was made with the blood-filter samples from Cutusuma, which had been stored for 10 years at 40 degrees C. While some deterioration of the blood-filter samples from Cutusuma had occurred over the 10-year storage period, no deterioration occurred with the Chijipata samples, which were stored for one year. Therefore, the method of collecting blood onto filter paper should prove useful for large-scale epidemiologic studies on human fascioliasis in the Bolivian Altiplano and in other regions where this disease is prevalent.