Studies were undertaken to determine the effect of repeated pure infestations with Boophilus microplus on susceptibility to subsequent pure infestations with Hyalomma anatolicum anatolicum, and the effects of pure infestations with both species of tick on susceptibility to a series of mixed infestations. Crossbred (Bos indicus X Bos taurus) calves were infested with Boophilus microplus (17 times), H. a. anatolicum (four times), followed by five mixed infestations of B. microplus and H. a. anatolicum. The decline in B. microplus engorgement from a mean yield of 274.4 +/- 60.3 ticks per host after the first exposure, to a mean yield of 9 +/- 4.6 per animal after the seventeenth exposure, was observed in animals exposed to only B. microplus. This might be due to acquired resistance. However, these animals were found to be as susceptible to H. a. anatolicum as animals which had never been exposed to ticks of either species. A decline in the yield of H. a. anatolicum from a mean yield of 92.1 +/- 10.7 after the first exposure to 54.7 +/- 11.3 after the fourth exposure, indicated that the cattle could also acquire resistance to repeated pure infestations with this species. After repeated pure infestations with both tick species, cattle reacted to five mixed infestations showing a high degree of resistance to B. microplus and low resistance to H. a. anatolicum (mean yield for B. microplus was only 10 +/- 8.1 ticks per host after the first mixed exposure and declined to 1.3 +/- 1.7 after the fifth, whereas the mean yield for H. a. anatolicum was 71.4 +/- 11.3 ticks per host following the first exposure and declined to 37.3 +/- 7.8 after the fifth). Host responses elicited to one species do not provide cross-resistance to the second species used in this study.