I mmunologic responses to bacteriophage ϕX 174 were studied in 26 patients with immunodeficiency diseases. In eight cases of infantile X-linked agammaglobulinemia, there was prolonged circulation of phage and no detectable antibody response. The remaining 18 patients cleared phage normally and produced antibodies. 10 of these patients made only IgM antibody in spite of repeated immunization; all of these have recurrent respiratory tract infections and require treatment with gamma globulin and antibiotics. Eight patients made both IgM and IgG antibody; they experience either milder or no infections, and only one requires treatment with gamma globulin.Prolonged circulation of bacteriophage ϕX 174 and the absence of a detectable antibody response appear to be distinguishing characteristics of X-linked agammaglobulinemia if severe combined immunodeficiency can be excluded.