Surface IgD on blood lymphocytes was studied in 10 normal adults and 24 patients with primary immunodeficiencies by direct immunofluorescence, together with surface immunoglobulins of the other classes and with spontaneous rosette formation with sheep erythrocytes. In the normal adults, 8% of the lymphocytes bore delta chains (the figures for mu chains being 11%) and, among the cells positive for mu and/or delta, 70% were mixed stained, 22% and 8% being single stained for mu and delta respectively. In 10 patients with sex-linked agammaglobulinemia or variable immuno-deficiency, practically no cells bearing surface immunoglobulins, including IgD, were detectable. A normal distribution of surface immunoglobulins, including the results of double labeling for mu and delta, was found in five other immunodeficiency patients in whom there was a block of the terminal differentiation of B lymphocytes into plasma cells. A new kind of block in the differentiation of the B cell line was observed in two patients affected with sex-linked severe combined immunodeficiency and variable immuno-deficiency respectively. They showed high figures for IgD-bearing lymphocytes, some of which carried simultaneously mu chains, contrasting with the absence of lymphocytes carrying IgM without IgD and of IgG- or IgA-bearing cells. The data obtained in several other patients with low figures for IgG- and IgA-bearing lymphocytes and a predominance of IgD-carrying cells with an excess of single producers for delta chains over single producers for mu chains suggest an analogous but incomplete maturation arrest.