Abstract 1. 1. The protein composition of echidna serum and milk whey were studied by the use of electrophoretic, immunological and gel filtration procedures. 2. 2. Serum separated on cellulose acetate electrophoresis into seven fractions with mobilities similar to the albumin, α 1-, α 2-, α 3-, β 1-, β 2-, and γ-globulin fractions of human serum. Six fractions were apparent in the whey, corresponding to the albumin, β 1- and γ-globulin fractions of serum, plus three fractions with α-globulin mobility. 3. 3. Whey was found to have a very high iron-binding capacity, 24–36 μg/ml, while the serum value was only 3–5 μg/ml. Three iron-binding components were shown in whey and one in serum by use of starch gel electrophoresis and autoradiography. The starch gel electrophoretic mobilities of the serum iron-binding proteins of different echidnas suggested two different genetic types. 4. 4. Gel filtration and immunoelectrophoresis of serum and whey showed the whey proteins to be of two types, serum-type and milk-specific. Whey albumin, γG-globulin and transferrin had similar molecular weights to the corresponding serum proteins. At least three milk-specific proteins of α-globulin electrophoretic mobility were found, one of large molecular weight and two of molecular weights less than serum albumin.