We showed that sera from normal subjects after antigenic challenge with intradermal PPD or Candida antigens or with subcutaneous tetanus vaccine contain a factor that blocks the binding of mouse monoclonal anti-Ia antibody to Ia-positive T cells or to B35 M cells, an Ia-positive human B cell line. The blocking activity appears 48 to 72 h after antigenic challenge and is gone by day 7. The appearance of the anti-Ia blocking activity coincided with a drop in the percentage of Ia-positive T cells and non-T cells in the peripheral blood of these subjects and also with a decrease in the density of surface Ia on the non-T cell population. The blocking was not genetically restricted; that is, serum from a given subject blocked anti-Ia binding to Ia-positive T cells of subjects with different DR haplotypes. The blocking activity was contained in the IgM fraction of the sera. The blocking activity of the sera was eliminated after absorption of the sera with Ia-positive but not with Ia-negative human cell lines. It would appear, therefore, that the blocking of monoclonal anti-Ia binding is caused by an IgM anti-Ia antibody that appears in normals after in vivo antigenic challenge.