THE heterogeneity of adult human haemoglobin is a well-established fact1–3. The present work was designed to determine if the slow electrophoretic fraction A2 has antigenic specificity and if this fraction can be detected immunologically in cord blood and in the presence of haemoglobin S and C. Previously described4 techniques were used for the preparation and the agar-gel electrophoresis of the haemolysates. Following electrophoresis the agar gel was cut into sections containing the haemoglobin fractions to be tested for antigenicity. These sections were then frozen, thawed and filtered through fritted Buchner funnels of medium porosity. The short duration (90 min.) and the use of large glass plates as the base of the agar gel made it possible to obtain quantities of eluates sufficient for immunization of rabbits. Eluates of the following fractions were used for this purpose: the main component (A1) of normal blood and of blood from a patient with thalassaemia minor; the respective A2 fraction and the non-haemoglobin fraction which we have previously4 designated ‘unidentified component 1’ and which probably is identical with Derrien's X 1 fraction2. In addition the most cathodic part of the fœtal haemoglobin from cord blood (Fig. 1b, area between brackets) was eluted from agar gel because of the possibility that this part contains A2 haemoglobin. Each eluate containing approximately 3 mgm. of protein was combined with an equal volume of Freund's adjuvant and injected into adult male albino rabbits on three or four different sites.