Filter-paper and starch electrophoresis and gel diffusion have been used to separate and identify the various protein components of tick extracts. Changes in the relative protein composition with increasing age of the eggs and larvae of Boophilus microplus (Canestrini) were revealed on filter paper electrophoresis. Toxicity of laboratory animals was due on most occasions to three fractions, F1, F2, and F6, in the egg extract, but to only one fraction, F2–3, in the larval extract. On a few occasions the a-globulin, F6 (haemixodovin), was non-toxic. The reason for the apparent variation in toxicity of this fraction is not understood. Toxicity decreased with age of both eggs and larvae, 14-day-old larvae being relatively non-toxic. Reactions of identity revealed the presence of similar or closely related components in the egg and larval extracts. Immunization with larval extracts prevented the toxic effects of the egg extracts. The antigen largely responsible for the development of skin hypersensitivity, F1, was also one of the toxic components. It was a y-globulin and had the greatest mobility of the y-globulins under these specified electrophoretic conditions. Skin-sensitizing activity was also shown by fraction F2, and to a lesser degree by F3. This may have been due to incomplete separation of the individual components.