In order to gain further information on the inheritance of serological characters in the hybridization of species,1 an attempt has been made to find an instance of fertile species hybrids where a serological differentiation of the parent types would be possible. A material answering these requirements consists of the two sorts of cavies mentioned in the title, the cross of which has been studied extensively by Detlefsen.2 As established by this author, the F1 females are fertile when mated with either of the parent species.When the blood and serum of the two species were allowed to interact, no hemagglutination or hemolysis took place. It was not difficult, however, to obtain hemolytic sera by several intraperitoneal injections of 0.5 cc. washed blood of the wild Brazilian guinea pig into the common domestic guinea pig. An experiment with one of the most active immune sera gave the following results:In all, 20 Brazilian and 23 common guinea pigs were examined. Whereas the cells of all the former reacted p...