We previously showed that anti-idiotypic mAb (mAb2) F16-16D7 (16D7) to the paratope (or paratope-related idiotope) of the anti-CD4 mAb HP2/6 induces anti-CD4 Abs in BALB/c mice. In view of the potential ability of 16D7 to induce anti-CD4 Ab in humans and the potential benefits of anti-CD4 Abs in the treatment of autoimmune diseases, we evaluated the immunologic response to and assessed the safety of four 2-mg 16D7 s.c. injections in one patient with systemic lupus erythematosus (SLE) and one with rheumatoid arthritis (RA). 16D7 induced anti-isotypic and anti-anti-idiotypic Abs (Ab3), which were almost exclusively of the IgG isotype. Ab3 specifically reacted with 16D7 as they inhibited its binding to mAb HP2/6. Although Ab3 did not react with cellular or recombinant CD4 (rCD4), single-cell enzyme-linked immunospot assays of anti-CD4 Ab production revealed many more spot-forming cells in rCD4- and 16D7-coated wells than in wells coated with BSA or 16D7 isotype-matched MK2-23. Spot-forming anti-CD4 Abs were specifically induced by 16D7, since rCD4-dependent spot formation 1) was not observed with PBL from one patient with SLE, one with mixed connective tissue disease, and one with melanoma immunized with MK2-23; and 2) was inhibited by 16D7 and not by MK2-23. Spot-forming anti-CD4 Abs recognize a CD4 epitope identical (or closely related) to that seen by HP2/6, since this specifically inhibited spot formation. A substantial, although transient, CD4+ T cell depletion was only observed in the RA patient. Local and/or general toxicity and laboratory and/or clinical signs indicative of immunodepression or diseases relapse were not observed during an 18-month follow-up.