Isoniazid (INH) is one among many drugs capable of inducing autoantibodies and, in some cases, a lupus-like syndrome (LE). A longitudinal study was performed in 24 tuberculosis patients treated with INH to detect antibodies (A-AH) to total histones and fractions. Antinuclear antibodies were observed in two patients after treatment. Higher frequency of IgM-AH was also observed. IgM-AH binding to all fractions were observed in those serum samples exhibiting stronger ELISA reactivity. Conversely, binding to only H1 occurred when lower IgM-AH activity was tested. Correlations with clinical expressions of LE were not observed in the present study.