The molecular pathogenesis of schistosomal liver fibrosis has been studied in a murine model of Schistosoma mansoni. A novel fibroblast growth factor, fibrosin, was identified as a product of a subpopulation of CD4+ lymphocytes resident in the hepatic egg granulomas. We describe a sensitive, quantitative, antigen-capture enzymelinked immunosorbent assay that can detect fibrosin in mouse serum and plasma at concentrations > 0.05 pg/ml. Using this assay, we detected in infected mice circulating fibrosin levels that were several fold increased above those detected in uninfected controls. Circulating fibrosin levels increased after week 4 of infection, reached a peak at week 8, and normalized by week 12. We propose that circulating fibrosin might be a useful marker of hepatic fibrogenesis in schistosomiasis.