In the double diffusion technique, antigen and antibody are allowed to diffuse toward each other and react in gel. The reaction may be carried out in agar plates (1) or in tubes (2, 3, 4). In the latter case three contiguous layers, the first consisting of antigen solution, the second of agar, and the third of antiserum or antiserum solidified with agar, are established in a tube (Fig. 1). Antigen and antibody molecules migrate into the central agar layer and combine there to form a visible band by precipitation. Preer (4) and Ouchterlony (5) have developed equations describing the position and movements of zones of precipitation. Their treatments are based on the assumption that the effect of the antigen-antibody reaction on diffusion may be neglected, and that precipitation occurs where equivalent concentrations meet. We have developed equations for the particular case of the stationary boundary which take into account this reaction.