We produced monoclonal anti-idiotypic antibody against immunoaffinity purified anti-U1-ribonucleoprotein (RNP) antibody from a patient (K.T.), by the cell fusion procedure. The specificity of monoclonal anti-idiotypic antibody (IgG1, kappa) was determined by inhibition studies. With the monoclonal anti-idiotypic antibody, cross-reactive idiotypes on anti-U1-RNP antibodies from unrelated patients with anti-U1-RNP antibody was detected in 57% of the samples. The anti-idiotypic antibody specifically suppressed the in vitro production of anti-U1-RNP antibody by lymphocytes from the patient K.T., and unrelated patients with a cross-reactive idiotype, in whom idiotype-reactive T cells were demonstrated. The results indicate that anti-idiotypic antibody may modulate the regulation of in vitro anti-U1-RNP antibody production.