The amino acid sequence of the first 35 residues of a low-molecular-weight protein obtained from the urine of rabbits treated with sodium dichromate was determined and shown to be identical with human beta2-microglobulin in 30 positions. Rabbit beta2-microglobulin, like the human protein, is strikingly homologous to the constant regions of rabbit immunoglobulin G, particularly the CH3 region.