It was shown that plasmid pUC19 DNA stimulates in vitro proliferation of CBA mouse splenocytes in a dose-dependent manner. Stimulation effect of the plasmid DNA is additive with COn A or LPS, synergistic with PMA and is inhibited by nonimmunogenic phosphodiester oligonucleotides and Fab fragments of antimouse Ig antibodies. These data and the data of affinity labelling of ODN-binding proteins indicate that immunoglobulin receptors are involved in DNA-induced lymphocyte activation.