In dog-fish spermatid nuclei two intermediate proteins S1 and S2 replace histones before the setting down of protamines. These spermatid-specific proteins were isolated by carboxymethyl-cellulose chromatography and purified by high pressure liquid chromatography. S1 and S2 are characterized by a high content of basic residues and by the lack of cysteine and phenylalanine. The determination of their amino acid composition and of their N- and C-terminal sequences prove that each protein corresponds to a specific molecule which can be considered neither as a histone hydrolytic product nor as a protamines precursor.