Twelve to 12-5-month-old Manchego sheep which had received a Haernonchus contortus infection with 2500 infective larvae (L-3) when they were 4-4.5 months old (Al,A2) and a challenge with 5000 L-3 2 months later (Al,A2,Bl,B2), finished in both cases with thiabendazole treatment, were subjected to a homologous infection with 7000 L-3 (A1,BI) or kept as control sheep (A2,B2). Fifty days after the infection the animals were treated with rafoxanide and challenged 18 days later with 14,000 L-3 of the parasite.Wide variations were seen, not observed at the 7000 L-3 infection, after challenge among the animals' faecal egg output. Retrospective analysis of the results obtained showed that the sheep response was apparently dependent on the extent of the contact with the parasite 6 months earlier and the elapsed period between priming and infection. A significant protective response was seen at challenge in the sheep which had received 2500 + 5000 L-3 (A2), suggesting the need of a period between 120 and 180 days without infection to show a strong response.No clear correlation was seen between parasitological results and the serum antibody response estimated by ELISA along the experiment and Western blotting analysis during the challenge. However, the less reactive sheep displayed lower ELISA values and simpler immunorecognition patterns in WB.