Anti-nuclear ribonucleoprotein (RNP) antibody titers were estimated serially in 19 anti-RNP antibody-positive patients, which included 10 patients with systemic lupus erythematosus (SLE), 6 patients with mixed connective tissue disease (MCTD) and 3 patients with rheumatoid arthritis (RA). Counterimmunoelectrophoresis was used for antibody detection and a calf thymus extract was utilized as an antigen. Anti-RNP antibody titers in most patients with MCTD and RA did not fluctuate significantly, whereas those in patients with SLE fluctuated by more than a four-fold serum dilution. The anti-RNP antibody titers in SLE seemed to correlate with disease activity. Previous reports have stated that anti-RNP antibody titers do not fluctuate easily throughout the disease course. The present report shows that they do fluctuate in patients with SLE. The use of high dose of steroids in SLE might be responsible for the fall in antibody titer.