The bone collagens in seven different species of Pleistocene mammals were analyzed for their amino acid compostions. Approximately 70per thousand of demineralized proteinaceous matter in compact bones and 13per thousand in spongy bones were recovered as pure collagen by treatment of the demineralized bone with CaCl₂ solution. A higher yield, approximately 90per thousand, was obtained when the demineralized matter from compact bone was treated with neutral buffer solutions. With the exception of higher amounts of alanine and smaller amounts of hydroxyproline in some specimens, amino acids are present in similar amounts in the fossil collagens and in those moderan mammals, but such minor differences may be significant.