Chigger infestation is often associated with severe cutaneous reactions. Mice were given four infestations with the pest chigger Eutrombicula cinnabaris, and each exposure was separated by a 14-day mite-free period. Mean duration of engorgement was nine to ten days for a first exposure and four to five days for a fourth exposure. An initial exposure did not elicit macroscopic changes at chigger attachment sites, while all third and fourth exposure animals had marked reactions consisting of erythema, epidermal thickening and serous exudation. Approximately 20% of second exposure animals had macroscopic changes at chigger feeding sites, but these reactions were much less intense than the responses of third and fourth infestation hosts. Third and fourth exposure animals had infiltrates of lymphocytes, eosinophils, basophils and neutrophils at attachment sites, with eosinophil influx the most intense. Cutaneous reactivity to chigger feeding was adoptively transferred with lymphocytes from fourth exposure animals. Passive transfer of serum from fourth infestation donors resulted in heightened reactivity to a challenge infestation. Skin testing, after a fourth infestation, with an extract of whole E. cinnabaris larvae provided evidence for Arthus and delayed type hypersensitivity responses to chigger antigens. Chigger-reactive homocytotropic antibody was not detected by skin testing and active cutaneous anaphylaxis.