Naive and actively and passively sensitized guinea pigs were infested with adult Amblyomma americanum ticks. Naive hosts allowed for 75-81% tick yield, 518-581 mg mean tick weight and an average tick feeding period of 12-13 days. Actively sensitized and challenged hosts, however, allowed only 47% tick yield (38.9% rejection), replete individuals were significantly reduced in weight (66.1% weight decrease) and required slightly longer time to feed (14 days). Passively sensitized hosts were given either serum or peritoneal exudate cells (PEC) harvested from actively sensitized and challenged guinea pigs which had expressed resistance. Recipients of immune serum allowed 56% tick yield (27.3% rejection), replete individuals had a mean weight of 376 mg (35.3% reduction) and required an average feeding period of 15 days. However, recipients of immune PEC were conferred with stronger protection allowing only 50% tick yield (35.1% rejection) with a mean tick weight of 413 mg (28.9% reduction) and females required an average feeding period of 15 days to become replete. Cutaneous cellular responses to tick feeding in naive animals were dominated by neutrophils in both female and male attachment sites (62-64%), with the cellular response (lesion) in female sites being 10 times that of male sites. In actively sensitized and challenged (resistant) hosts, cutaneous cellular responses to both females and males were dominated by basophils (53-59%), and lesion size was nearly 20 times greater than in primary feedings, indicating heightened sensitivity to tick bite.