Fourteen antigenic constituents have been detected in Arachis hypogaea seeds. The major proteins of the classic arachin and conarachin fractions have been identified. Arachin contains 4 antigens (the major one called alpha-arachin) and conarachin contains 2 which have been called alpha(1), and alpha(2)-conarachin. Structural differences between alpha-arachin, alpha(1) and alpha(2)-conarachin are indicated by their different antigenic specificities. alpha-Arachin precipitates as a separate entity at low temperature. The action of trypsin on this protein induces an increase in electrophoretic mobility and prevents precipitation at low temperature. This enzyme has no detectable effect on alpha(1) and alpha(2)-conarachin.The proteins of the cotyledon and of the entire axis are quite similar in their immunoelectrophoretic patterns. However, quantitative and qualitative differences occur between the proteins found in the cotyledon and those of the 1-millimeter tip of the axis. In the latter, there is a 4-fold smaller proportion of alpha-arachin than in the cotyledon. Immunoelectrophoretic analysis of subcellular preparations from cotyledons confirms that alpha-arachin is an aleurin and that alpha(1)-conarachin is a typical cytoplasmic protein. Large and small aleurone grains appear very similar in their qualitative antigenic composition.