H‐2 alloantisera contain Ia antibodies whose detection is obscured by the presence of serodominant K, D specificities. A method is described whereby Ia antisera can be prepared by absorbing conventional H‐2 sera with donor‐strain erythrocytes. The analysis of such absorbed antisera has led to the identification of the new specificity Ia. 16, and the redefinition of the specificities H‐2.34 and H‐2.46 as complex Ia specificities which map in the I‐A region of the H‐2 complex. A number of other so‐called H‐2 specificities also appear to have I