Summary Infection of mice, rabbits, and guinea pigs with a number of variants of Brucella abortus (clones of S, I, R, M, etc. isolated from virulent and avirulent cultures) was accompanied by strikingly different agglutinin production among the three host-species after inoculation of identical antigens. All non-smooth types, as well as the smooth type, could be recovered from the spleens of mice which had been infected with the respective type five weeks previously; no similar recovery of non-smooth types could be obtained after culturing of the spleens from guinea pigs and rabbits infected with non-smooth variants, the inoculated material could be recovered only from those animals which had been infected with the smooth type. This result supports the validity of in vitro observations which had demonstrated a differential selective action of normal sera from certain species upon the establishment of non-smooth types in broth-cultures. Results obtained during the infection experiments also revealed differences in antigenic properties between certain variant clones of identical colony type, thereby confirming previous observation which had shown such differences in vitro with the help of an acriflavine test. The effect of antisera, produced from smooth types and several non-smooth variants, in mice previously infected with a virulent smooth type was tested; on the basis of spleen-weights, the greatest mitigation of infection was produced in animals treated with antiserum produced from an “Intermediate” variant. The relation of these results to such problems as species resistance, testing of antigenic properties, isolation of variant types from infected hosts, as well as the dangers involved in generalizing from results obtained with one host-species only, has been discussed.