Twenty-nine rabbits were exposed to 4 infestations with 20, 50, and 100 adult red-legged ticks (Rhipicephalus evertsi evertsi). Upon challenge, 16 rabbits (55%) contracted paralysis, and 14 of these (88%) died. Most deaths occurred in rabbits challenged with 100 ticks. Immunity to paralysis and subsequent resistance to tick feeding developed in the hosts that survived the initial challenge. Antibody titers to a salivary gland antigen increased progressively as determined by the following tests: passive hemagglutination, agar gel double immunodiffusion, and enzyme-linked immunosorbent assay. Rabbits challenged with 20 ticks displayed declining titers with advanced frequency of challenge and few of the rabbits in this group developed paralysis. There was no decline in antibody titers in groups of rabbits exposed to 50- and 100-tick loads. It is suggested that a certain quantum of antigen injected by the ticks is necessary for induction of immunity to paralysis.