In the course of inquiry into the mechanisms of elaboration of oviductal fluid in the rabbit, protein patterns of tubal fluid have been compared with those of serum. Proteins in fluid, collected from normal estrous rabbits by oviduct cannulation, and in serum from the same animals were analyzed by means of analytical acrylamide gel disc electrophoresis stained for protein and carbohydrate, Ouchterlony double-immunodiffusion, and acrylamide gel immunophoresis. The results of these studies were mutually supportive and revealed that: (1) Several protein components are common to oviducal fluid and serum. However, certain of them are present in different proportions in these two body fluids, e. g., transferrin and pre-albumin are found in far greater quantity relative to albumin in tubal fluid than in serum. (2) Many serum proteins have no counterparts in oviduct fluid. (3) Conversely, several proteins are unique to oviducal fluid, in that they are undetectable in serum; one such specific protein, a glycoprotein, migrates to the γ-globulin region of acrylamide gels; another, a non-PAS staining protein, migrates to the post-albumin region and constitutes approximately 5% of the total protein of oviduct fluid. The presence of only certain serum proteins in oviducal fluid and the disparity in the relative amounts of these common proteins in the two body fluids suggest that certain serum proteins may be selectively transferred into the oviducal lumen. The presence of unique proteins in tubal fluid implies that oviducal cells may be engaged in specific secretory activity.