Lymphocyte proliferation and skin hypersensitivity responses to parasite antigens were measured as indicators of cell-mediated immunity in genetically resistant and random-bred lambs following infection with Haemonchus contortus. Responses of PBMC to the mitogens PHA, PWM and LPS were also assessed to determine if infection was associated with suppression of general immune function. All lambs demonstrated significant antigen-specific lymphocyte proliferation prior to experimental infection. The responses did not differ between resistant and random-bred lambs and the correlation between pre-infection stimulation indices and resistance to challenge infection was also not significant. Following infection, PBMC from resistant lambs consistently exhibited higher blastogenic responses to both larval and adult antigens than PBMC from random-bred lambs. Addition of cyclosporin A to the antigen-stimulated cultures resulted in significant inhibition of blastogenesis, suggesting that the cells responding to parasite antigens were predominantly of the T-helper cell phenotype. Proliferation responses of resistant and random-bred sheep were similar for all mitogens tested. However, a transient suppression of lymphocyte reactivity to PHA and PWM was noted in all infected lambs. Responses to LPS remained unaffected during infection. There was no difference in the immediate hypersensitivity responses to intradermal inoculation of parasite antigens between the genotypes. However, the delayed-type hypersensitivity responses to parasite antigens were significantly greater in resistant lambs than those in random-bred lambs. Together, these data suggest that resistant lambs have an enhanced ability to mount parasite-specific cell-mediated immune responses when compared with random-bred lambs and that this may contribute to differences in the susceptibility of two genotypes to haemonchosis.