Changes in the fluorescence spectra and quantum yield of 1-pyrenebutyric acid (1) induced by human, bovine, rabbit, and dog serum albumins and healthy adult serum, liver-disease patient serum, and renal-disease patient serum were compared at pH 7.4. Human serum albumin, healthy adult serum, and liver-disease patient serum caused marked quenching of the fluorescence of 1-pyrenebutyric acid. On the contrary, bovine, rabbit, and dog serum albumins increased the fluorescence intensity. Renal-disease patient serum also enhanced the intensity but caused quenching of the fluorescence after treatment with charcoal. Thus, 1-pyrenebutyric acid clearly shows interspecies differences in the environment or the structure of the binding site on serum albumin between humans and other animals. Furthermore, it is suggested that some substance which influences the character of the binding site for 1-pyrenebutyric acid exists in the serum of renal-disease patients.