Once you have seen the spectrum of one protein you have seen the spectra of all pro teins. Or so it would seem. While the general characteristics of the absorption curve may appear to be similar for all proteins (i. e. , in acid and neutral solution there is a minimum at 250 nm, a maximum at 278-282 nm, and no absorption above 310 nm; in alkaline solution the maximum and minimum shift to longer wavelengths), there are subtle differences which can be seen when the spectra of many proteins are compared. It is these differences which reflect changes in amino acid content and in the milieu in which the protein has been dissolved. The spectra in this book provide samples of these subtle spectral differences and permit comparisons to be made. This book was prepared to have its index read and its contents referred to. For the reader who desires to know what a protein spectrum looks like in acid and alkaline media, after X-ray or UV irradiation, or after photo-oxidation or B-bromosuccinimide treatment, spectral representations of all these experimental situations and many others are available. The indicies were prepared to provide the maximum information with the minimum effort. In addition to an alphabetical listing, all spectra are referred to by species, tissues, and the organs from which they were taken. There are also "environmental" indicies related to the treatment the proteins received prior to having their spectra taken. Technical information concerning instrumentation is lacking.