A solid phase enzyme-linked-immunosorbent assay (ELISA) for the determination of antibodies against denatured, single-stranded (ss-) DNA is described. Polystyrene cuvettes coated with ss-DNA were incubated with serum samples and the anti-ss-DNA antibodies bound were detected by means of an anti-IgG-alkaline phosphatase conjugate. The binding of anti-ss-DNA antibodies in individual sera was expressed as units calculated as % of the absorbance in relation to the absorbance value obtained with a reference pool. Absorption experiments showed that the assay is specific for antibodies against denatured DNA. By using immunologically purified anti-ss-DNA antibodies the assay was shown to detect specific antibodies in concentrations down to 1 ng/ml. Antibodies against DNA could be detected in 94% of sera with antinuclear antibodies.