Abstract.Acquired resistance to adults of the two‐host tickRhipicephalus evertsi evertsiNeumann has been demonstrated in guinea‐pigs and rabbits. Four infestations of both hosts withR.e.evertsiadults resulted in a significant reduction in the mean weight of the engorged females. A decline in the mean weight of the female ticks was also observed in ticks which fed on a rabbit previously injected with serum taken from a rabbit resistant toR.e.evertsi.This suggests that humoral immunity may have an important role in the acquired resistance to ticks. In both hosts an increase in the level of serum globulins was recorded. In rabbits the level of gamma globulin was affected, while in guinea‐pigs an increase was observed in the concentration of α and β globulins. The differences in host responses to theR.e.evertsiadults are di