Basophils infiltrate the skin and other tissues as part of T lymphocyte and/or antibody-mediated immune responses to certain protein antigens, viruses, tumors, and parasites. Although basophils may comprise a significant fraction of leukocytes in these reactions, their precise role has been poorly understood. Guinea pigs expressing acquired immunity to the tick Amblyomma americanum develop basophil- and eosinophil-rich cutaneous inflammatory responses at tick feeding sites, and tick rejection is associated with extensive local basophil degranulation. We report that a specific anti-basophil serum (ABS) eliminate basophils at tick feeding sites and abolished immunity to A. americanum. ABS does not react with eosinophils and did not reduce blood or bone marrow eosinophils; however, ABS-treated animals had diminished eosinophils at tick feeding sites, perhaps because these sites were deficient in basophil-derived eosinophil-chemotactic factors. AES treatment markedly reduced feeding site eosinophils, did not affect basophil levels, and partially impaired tick resistance. Taken together, our experiments suggest cooperation between basophils and eosinophils in the expression of immunity to ticks in guinea pigs.