A simple but sensitive radioimmunoassay method for the quantitation of IgD in sera and body fluids has been developed. Specifically purified chicken anti-delta was covalently coupled to cyanogen bromide activated filter paper discs and the discs were incubated together with the samples to be analyzed. The washed discs were next incubated with 125-I-labeled specifically purified chicken or rabbit anti-human delta-antibodies. IgD on the disc was quantified by reference to a standard curve prepared with purified human IgD. The sensitivity of this assay was approximately 10 ng/ml IgD. IgD was demonstrable in cord sera (18 of 18), hypogammaglobulinemic sera (15 of 15), amniotic fluids (6 of 6), salivas (8 of 10), normal cerebrospinal fluids (12 of 13) and cerebrospinal fluids from patients with subacute sclerosing panencephalitis (5 of 5). The finding of IgD in human secretions raises the interesting possibility that IgD may be a secretory antibody.