Cathepsin L1 (CL1), an immunogenic cysteine proteinase secreted by juvenile and adult Fasciola hepatica, was assessed for its potential as a diagnostic agent for the serologic detection of human fascioliasis. Using ELISAs, we compared the ability of liver fluke homogenates (LFH), excretory/secretory (ES) products, and CL1 to discriminate between seropositive (infected) and seronegative (noninfected) individuals within a population of 95 patients from the Bolivian Altiplano. A high prevalence of human fascioliasis has been reported in this region. The division between the seropositive and seronegative individuals was poorly defined when LFH was used as the antigen. A greater discrimination between these populations was achieved with both ES and CL1. A K-means cluster analysis using the combined ES and CL1 ELISA data identified a cluster of seropositive individuals. Cathepsin L1 detected a subset (20) of these seropositive individuals while ES detected all 26; however, ES detected nine additional individuals that were in the seronegative cluster. The ratio of the mean absorbance readings between seropositive and seronegative individuals was markedly improved by using conjugated second antibodies to IgG4, the predominant isotype elicited by infection. In these IgG4-ELISAs, CL1 again identified fewer individuals as seropositive than did ES, but improved the discrimination between the seropositive and seronegative individuals and thus provided a more conclusive diagnosis. Sera obtained from patients infected with schistosomiasis mansoni, cysticercosis, hydatidosis, and Chagas' disease were negative in these assays, which demonstrated the specificity of the IgG4-ELISA for detecting fascioliasis. Twenty of the 95 patients (21%) were seropositive for fascioliasis by the CL1 IgG4-ELISA, confirming the earlier reports of the high prevalence of disease in this region. A standardized diagnostic test for human fascioliasis, based on an ELISA that detects IgG4 responses to CL1, could be available to all diagnostic centers if sufficient quantities of recombinant CL1 can be produced.