Summary Fish anti-BSA differed from rabbit anti-BSA in at least three ways. 1) The fish antibody had three equivalence points, 50, 25 and 6.25 mg/ml, whereas rabbit antibody had only one, 0.31 mg/ml. 2) The fish antibody had a larger amount of low-avid antibody. 3) The fish antibody migrated in the same direction but more slowly. However, it could not be demonstrated that fish antibody has a higher cross-reactivity than rabbit antibody. The modified agar gel diffusion method for demonstrating the equivalence point of an antigen-antibody system is recommended as a useful method. As little as 0.75 ml of either antibody or antigen is all that is required to determine the equivalence point.