Guinea-pigs infested with male ticks of the species Rhipicephalus appendiculatus, and rabbits infested with R. evertsi evertsi, acquired immunity to conspecific female ticks. The hosts were first infested with male ticks and thereafter were challenged with males and females of the same species. The mean weight of the engorged females of R. appendiculatus fed on guinea pigs previously infested with male ticks was 509.0 (+/- 41.4) mg compared with that of females fed on control guinea pigs (651.2 +/- 31.8 mg). Similar weight differences were observed for R.e. evertsi females which fed on rabbits previously infested three times with male ticks. The mean weight of the female ticks which fed on these rabbits was 520.1 (+/- 29.8) mg compared with 640.7 (+/- 30.2) mg of R.e. evertsi females which fed on control hosts. The concentration of gammaglobulins in the sera of rabbits was monitored at various intervals after the first infestation. It was found, for the first time, that infestation of laboratory animals with male ticks conferred immunity, but to a lesser degree than infestation with both sexes. It was also shown that the level of gammaglobulins increased from 3.4 +/- 0.28 g l-1 to 7.3 +/- 0.24 g l-1 in sera of rabbits hosts as a result of the feeding activity of males, but to a lesser extent than in sera of rabbits on which both sexes had fed (10.8 +/- 2.4 g l-1).