Immune bodies were produced by the injection of cows with living or formalinkilled Trichomonas foetus. The degree of antibody-response was low, as determined by the test described above, and results indicate that T. foetus is not highly antigenic. In the sera of five heifers injected intramuscularly with washed motile trichomonads, 2 heifers with formalin-killed trichomonads, and 2 heifers intravenously with living trichomonads, immobilization of the microorganisms was observed in dilutions ranging from 1:4 to 1:128. Thus, there there was some degree of variation of titer among the cows. Inoculation with a single injection of trichomonads after a rest period of approximately 102 days produced titers as high as the previous maximal titer or higher. The average time for the agglutinins to appear after the first injection was 17 days, for maximal immobilization 55 days, for total duration of the agglutinins 177 days and duration of complete immobilization 40 days. No agglutinins were detected in the sera of 400 normal trichomonad-free cows and 52 normal trichomonad-free bulls. Also, this test failed to show agglutinins in 11 trichomonad-infected bulls. Sera from 51 cows divided into 3 groups according to the type of trichomonad-infection were tested. In 10 cows in which trichomonads were recovered from the vaginal tract all sera were negative. In 12 cows with early abortion produced by T. foetus, only 2 sera reacted, one with a complete immobilization at a dilution of 1:1, and one with a partial immobilization at the same dilution. In the 29 cows with trichomonad-pyometra only 2 sera reacted slightly, both with partial immobilization at a 1:1 dilution. On the data presented with 9 injected animals, 400 normal cows, 52 normal bulls, 11 infected bulls, and 51 infected cows, it appears that the immobilizationreaction cannot be successfully utilized as a means for the diagnosis of bovine trichomoniasis.