The authors first discuss the principle of the Treponema pallidum immobilization (TPI) test, considered both as a qualitative and as a quantitative test, and then various specific aspects of passage of the Nichols strain of treponeme in rabbit testes. A number of important technical details and modifications in the test procedure are then reviewed, and the clinical value of the test is discussed.The sensitivity of the TPI test is considerably greater than that of the serological tests for syphilis more usually performed, and its specificity is also remarkably high. However, it is stressed that, at the present stage of development, the greatest value of the TPI test lies in its use as an excellent aid to diagnosis rather than as a test of cure.