Summary For the first time, so far as is known, the theory of Neurath has been applied experimentally to the determination of the diffusion coefficient of one component of a binary mixture in the presence of the other. The molecular weight of the saturated human albumin, horse anti-human serum albumin complex has been determined in the presence of antigen. The molecular weight so determined is consistent with previous findings that this antibody has a valence of two. In both the serum albumin and egg albumin systems the horse-antibody-containing complex has a significantly higher sedimentation constant than the corresponding complex containing rabbit antibody. The possible relationship of this finding to the distribution of combining sites on the antibody molecule is discussed.