A cryopreserved stabilate was prepared from Babesia bovis-infected Boophilus microplus ticks. The stabilate was used to infect a group of mature Bos taurus-Bos indicus cows. The survivors of the initial infection were rechallenged with the homologous stabilate 65 days later. All cows reacted parasitologically after primary challenge, and most responded serologically. With the exception of two cows, those not responding serologically died of acute babesiosis. The two that survived the primary challenge, but did not produce detectable antibody, died after secondary challenge. All serologically-positive cows survived the secondary challenge. Complement fixation and indirect fluorescent antibody tests were performed on all cows' sera and, in addition, immunoglobulin fractions were prepared and analyzed from selected sera. Both IgM and IgGl were important in the initial response. A secondary IgM response was noted by complement fixation assays. No activity was detected in any IgG2 fraction.