The primary and secondary serum antibody responses of Texel sheep to infective larvae (L3) and adult worms of Haemonchus contortus were studied. Ten-month-old sheep were infected with 20,000 H contortus L3, treated with ivermectin seven weeks later and, after four weeks, reinfected with 10,000 L3 once a week for six weeks. Faecal egg counts were significantly lower during the secondary infection than during the primary infection, but both infections induced antibody responses, as demonstrated by an enzyme-linked immunosorbent assay (ELISA). The primary antibody response developed rather slowly, but the secondary response developed more rapidly and the IgA responses against L3 antigens and the IgG1 and IgG2 responses against adult antigens were twice those observed during the primary infection. These accelerated and enhanced responses after the reinfection suggest an immunological memory for H contortus antigens.