Blood leukocyte changes, serum hepatic enzyme levels, lymphocyte proliferation in response to Concanavalin A (ConA) and to parasitic excretory-secretory products (FhESP), and antibody (IgG and IgM) responses (ELISA and Western blot) were studied in sheep, the natural susceptible host of F. hepatica, during the first 3 months of an experimental primary or secondary infection. The proportion of flukes established was similar in once- and twice-infected groups, but the flukes originating from the secondary infection migrated more rapidly to the bile ducts. Primary infection induced a marked peripheral eosinophilia from 3 to 13 weeks post-primary infection (PPIW). FhESP-specific IgM were produced from PPIW 2 with peaks in PPIW 3 and 9-10; FhESP-specific IgG increased from PPIW 2 to 6 and became stable afterwards. Western blotting revealed 12 major antigenic fractions in FhESP from 12, 15, 20, 24, 27, 28.5, 30, 41, 51, 56, 69 and 156 kDa; some non-specific ones have been characterized. A sequential recognition of higher then lower molecular weight antigens was observed. FhESP-specific lymphocyte proliferation was marked from PPIW 2 to 5. In contrast, ConA stimulation of lymphocytes was decreased. After secondary infection in PPIW 6, immune responses were modified. The ConA-induced lymphocyte proliferation was transitorily increased. In contrast, the humoral response, in particular against the early recognized antigens, and the level and the duration of the FhESP-specific lymphocyte proliferative response, were reduced.