Immune resistance to infestation by an ixodid tick, Rhipicephalus appendiculatus, the vector of the African cattle disease, East Coast Fever, was induced in rabbits by either repeated tick feeding or immunization with tick extracts. In addition to resistance to tick infestation, tick extract immunization led to a reduction in the viability of eggs laid by ticks feeding on the immunized host. Resistance to infestation by ixodid ticks has previously been reported by others to have a humoral immune component. Therefore, antibodies from resistant host animals were used to detect the tick antigens they recognized as an approach to identification of the target antigen(s) for the observed immune responses on feeding ticks. In crossed immune electrophoresis two antigens were detected using sera from animals made resistant by multiple tick infestations. Sera from extract immunized animals detected these antigens and nine others. The tick antigens detected by both sets of sera in crossed immune electrophoresis were radiolabelled with [35S]amino acids. No labelled antigens were detected by Staphylococcus aureus mediated immune precipitation with sera from hosts made resistant by multiple infestations. Antibodies from extract-immunized animals identified nine protein antigens by S. aureus immunoprecipitation. The molecular weights of these antigens as assessed by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis were 180,000; 140,000; 130,000; 98,000; 92,000; 88,000; 85,000 and 82,000. The rate of synthesis of these antigens appeared to vary in relation to the tick feeding cycle.