To investigate the mechanism of the long-lasting efficacy of chimeric monoclonal anti-TNFalpha antibody (cA2) therapy for rheumatoid arthritis (RA), eight patients with refractory RA were treated with a single infusion of cA2 and the changes in circulating cytokines (IL-1, IL-6, TNFalpha, and IL-10), soluble cytokine receptors (TNF-RI, RII, and sIL-6R) and peripheral white blood cell (WBC) subset counts were followed up long-term (12 weeks) after cA2 therapy in them. Significant clinical responses (>20% improvement according to Paulus' criteria) were observed just after cA2 infusion and lasted more than 4 weeks in all patients, as reported elsewhere. Moreover, five of the eight patients showed prolonged clinical responses (>12 weeks). The elevated serum IL-6 and sTNF-RI (or RII) levels before treatment rapidly decreased after treatment. The serum IL-10 levels also significantly elevated before treatment. The elevations of serum IL-10 levels were augmented after treatment and stayed higher than the baseline in four patients with prolonged clinical responses. No significant TNFalpha, IL-1alpha and -beta, or sIL-6R were detected in the sera of the patients before treatment and during the whole study period. On the other hand, peripheral lymphocytes as well as total WBC and neutrophils increased for 4 weeks after treatment. However, thereafter, only the lymphocyte count decreased gradually and stayed below the baseline long-term (12 weeks). FACS analysis revealed the predominance of T lymphocytes in the decrease in lymphocyte counts. These results suggest that the augmentation of IL-10 production and the decrease in T cells might partly contribute to the long-lasting efficacy of cA2 treatment in RA.