A large number of proteins were tested for the property of intrinsic phosphorescence in deoxygenated aqueous solution at room temperature. The majority of proteins exhibit phosphorescence under normal solution conditions. Phosphorescence lifetimes from 0.5 millisecond to 2 seconds were observed in three-fourths of the proteins tested. The lifetime appears to correlate with relative isolation of the tryptophan indole side chain from solvent. With few exceptions, proteins in general can be expected to display a phosphorescence lifetime greater than 30 microseconds. This widespread characteristic of proteins has been largely overlooked because long-lived phosphorescence is highly sensitive to quenching by low levels of dissolved oxygen in solution. Protein phosphorescence offers a new time domain and a far wider dynamic range than has been used before for photoluminescence experimentation.