The sera of 51 patients with active, untreated pulmonary tuberculosis were examined for the presence of a common anti-DNA idiotype, 1616. Thirty-four of the 57 sera (60%) had an increased level of the idiotype, as measured by enzyme-linked immunosorbent assay using a specific rabbit anti-1616 serum. Of 28 matched control sera, only 1 (4%) was found to be positive for the idiotype, The sera of patients with tuberculosis also showed increased activity against a variety of antigens with which lupus autoantibodies are known to crossreact (e.g., single-stranded DNA, double-stranded DNA, polynucleotides, and cardiolipin). A correlation was observed between serum IgG and IgM levels and the 1616 idiotype levels.