Classical antibody deficiency syndromes, such as sex-linked aggammaglobulinemia, are rare and relatively homogeneous in presentation. In the present investigation an unselected group of 3,213 individuals from a community health study was examined in an attempt to estimate the prevalence of the commoner and largely unclassified examples of immunoglobulin deficiencies defined by the lower 2.5 per cent of the population. The prevalence of selective IgA deficiency (an isolated absence of IgA) was 0.097 per cent and that for selective IgM deficiency was 0.03 per cent. No isolated absence of IgG was found. In addition to these deficiency syndromes, concentrations of each of the immunoglobulins were found to be highly correlated to each other.