Cells from adult Fasciola hepatica were fused with cells from a murine BALB/c myeloma Sp2 line. The hybrid cells were grown in HAT (hypoxanthine, aminopterin, and thymidine) medium, cloned and subcloned, and shown to express parasite antigen for 1 year after fusion. Expression of parasite antigen was demonstrated by the following: 2 histogram flow cytometric analyses, in which a population of hybrid cells in the population of 7 month cultured hybrid cells showed 57% more fluorescence when treated with an anti-F. hepatica serum followed by anti-rabbit immunoglobulin G coupled to fluorescein isothiocyanate as compared with the same hybrid cells washed and treated with normal rabbit serum; Sp2 myeloma cells treated with an anti-F. hepatica serum or normal rabbit serum followed by fluorescein-labeled anti-rabbit IgG had the same negative fluorescence; BALB/c mice immunized with PBS-washed cells from a subclone of these hybridomas developed anti-F. hepatica antibodies (shown by the Falcon assay screening test enzyme-linked immunosorbent assay); and antibodies recognized an F. hepatica antigenic polypeptide of 57,000 Mr in a Western immunoblot. These helminth:myeloma hybrids expressed murine host markers, further confirming the hybrid nature of this cell line. F. hepatica cells alone, like their Sp2 fusion partners, die in HAT supplemented medium by 9 days of culture. F. hepatica:Sp2 hybridomas have been grown continuously in HAT medium for > 1 year.