Filarial infection is characterized by an immune response associated with the production of Ag-specific IgG4 and IgE and IL-4 and IL-5. To identify filarial Ags capable of inducing such responses and to analyze the role Ags themselves play in sustaining it, 24 recombinant filarial parasite proteins were screened for their ability to be recognized by sera from 67 individuals with tissue-invasive filarial infections. Among the recombinant proteins that were recognized by IgG4 or IgE Abs in 25% of the sera or more, two were selected on the basis of their ability to elicit polyclonal and Ag-specific IgE/IgG4 Abs in vitro. Ov27 (analogous to Ov7/cystatin, a cysteine protease inhibitor) and OvD5B (analogous to Ov33, an aspartyl protease inhibitor) induced both a polyclonal and Ag-specific IgE/IgG4 response that was blocked by neutralizing Abs to IL-4 and to IL-13 or by soluble IL-4 receptors. Recombinant human IFN-gamma and IL-12 also led to a decrease in the production of polyclonal and Ag-specific IgE/IgG4 Abs. In addition, these two recombinant proteins preferentially stimulated the secretion of IL-4, IL-5, and IL-10 (in contrast to IFN-gamma). The data suggest that certain epitopes on filarial Ags preferentially elicit a Th2-type response and provide an in vitro model for dissecting the mechanisms underlying this preferential response.