SYNOPSIS The haptoglobin phenotypes of 3,332 individuals, consisting of 2,930 caucasians and 402 negroes living in the greater Boston area, were determined. Of these, 3,222 were hospitalized medical patients fully documented regarding diagnoses. One hundred and twentyeight of the total population studied were shown to exhibit starch gel anhaptoglobinaemia (3 7 %). Re-evaluation on acrylamide gel of 118 0-0 samples revealed that the majority (94 %) were derived from patients exhibiting hypohaptoglobinaemia rather than anhaptoglobinaemia.Haptoglobins are (X2 glycoproteins which form stable complexes with free haemoglobin (Polonovsky andJayle, 1939 and1940). Smithies (1955) using starch gel electrophoresis demonstrated three distinct serum haptoglobin phenotype patterns. These were designated 2-1, 2-2, and 1-1, and are the expression of three genetypes produced by a single pair of allelomorphic genes (Smithies and Walker, 1956). Later, it was observed using the starch gel technique that sera from a small percentage (< 3 %) of healthy adult caucasians exhibited a total absence (anhaptoglobinaemia) of serum haptoglobin proteins (Allison, Blumberg, and Ap Rees, 1958). Family studies have demonstrated that in some instances such a phenomenon can be explained on a genetic basis (Harris, Robson, and Siniscalco, 1958;Matsunaga, 1962).Illnesses accompanied by intravascular haemolysis (Brus and Lewis, 1959;Owen, DeGruchy, and Smith, 1960;Reerink-Brongers, Prins, and Krijnen, 1962) and/or liver disease with hepatocellular dysfunction (Jayle and Boussier, 1955;Owen, MacKay, and Got, 1959) Received for publication 16 February 1970. level below the accepted normal range of 40 to 180 mg %, expressed as haemoglobin-binding capacity (Nyman, 1959; Bayani-Sioson, Lauch, Sutton, Neel, Home, and Gershowitz, 1962). If depletion is severe (ie, haemoglobin-binding capacity of less than 20 mg %) then electrophoresis on starch gel may show an apparent absence of haptoglobin-haemoglobin protein complexes (anhaptoglobinaemia). Such sera are classified 0-0 phenotype. By using either a freezedrying or ultraffiltration technique as a means of concentrating serum proteins, investigators (Whitten, 1961;Murray, Robinson, and Visnich, 1966) using starch gel have been able to determine the haptoglobin phenotype of apparent anhaptoglobinaemic sera. The true haptoglobin phenotype of most sera classified 0-0 (starch gel) can, however, be ascertained without prior concentration using an acrylamide gel electrophoresis technique (Nandi and Lewis, 1970). The present paper reports a study of the haptoglobin phenotype distribution within a hospital population and the result of a re-examination on acrylamide gel of 117 patients with the 0-0 starch gel haptoglobin phenotype classification.