In this report, we show that calf thymus histone 1 stains metachromatically with Coomassie Brilliant Blue R-250. Histone 1 gel bands appear red instead of the more familiar blue color characteristic of the vast majority of proteins. The red color and the spectral properties of histone 1 bands are qualitatively similar to those of collagen and procollagen bands which, as previously shown by others, also stain metachromatically (Micko, S. and Schlaepfer, W.W. (1978) Anal. Biochem. 88, 566-572 and McCormick, P.J., Chandrasekhar, S. and Millis, A.J.T. (1979) Anal. Biochem. 97, 359-366). In contrast to histone 1, histones 2A, 3 and 4 stain blue; histone 2B also stains predominantly blue, but with a faint red tint. Both red and blue bands display an absorption maximum in the vicinity of 560 nm, but red bands display an additional maximum in the vicinity of 530 nm. There are quantitative differences between the red bands; the 530 nm peak is more prominent in the spectrum of the collagen band than in the spectrum of the histone 1 band. The spectra of the blue bands are all very similar except that the spectrum of the histone 2B band is shifted slightly toward lower wavelengths. To account for the spectral differences between protein bands, we propose a model in which closely-spaced proline residues are responsible for the chromotropic effect. Localized concentrations of proline residues are present in both collagens and histone 1 and a small cluster is also present in histone 2B. © 1980.