A study was made of the ultrastructure of L. icterohaemorrhagiae and the microagglutination reaction and lysis, depending on the time of action, the amount of antibodies and the presence of complement. The primary site of activity of specific antibodies separately or with complement is the external membrane. The presence of specific antibodies alone caused a disturbance of permeability of the barrier of the external membrane and a change in the osmotic pressure within the organism; death of Leptospirae depends on the time of contact with specific antibodies. With addition of complement the ultrastructural intactness of the external membrane is disturbed; due to this there is successive lysis of the external membrane, and of the cytoplasmic cylinder, and cell death.