From the adult lens of Triturus viridescens, a fraction of proteins was isolated which corresponds to gamma-crystallins of higher vertebrates. Tests by immuno-electrophoresis indicate that the antiserum against this fraction reacts with gamma-crystallins, but not with alpha- or beta-crystallins. With this antiserum, an immunofluorescent reagent has been prepared for detection of gamma-crystallins from newts. In the normal lens of the adult newt, these crystallins are detected in fiber cells and fiber material, but not in the epithelial cells. During transformation of the iris into the lens after lens removal, the staining reaction is negative in the regenerating tissue up to the time the prospective primary fiber cells begin to elongate. Subsequently, without exception those cells in fiber differentiation indicate a gamma-crystallin reaction. When the secondary fiber cells are produced at the equatorial zone of the regenerating lens, they also begin to show a gamma-crystallin reaction. Thus, gamma-crystallins characterize fiber differentiation.