Serum cytokines such as interleukin 1 beta (IL-1 beta), interferon gamma (IFN-gamma), and tumor necrosis factor alpha (TNF alpha) were measured in 40 patients with rheumatoid arthritis (RA). In the 40 patients studied, serum IL-1 beta was detected in 5 patients, IFN-gamma in 10 patients, and TNF alpha in 20 patients. The IL-1 beta-positive group showed increased values of activity indices compared to the IL-1 beta-negative group. Values of serum IFN-gamma correlated well with the number of peripheral blood lymphocytes and CD3+ cells and with the percentage of CD3+ CD26+ cells. Values of serum TNF alpha correlated positively with the number of peripheral blood monocytes and the percentage of CD3+ HLA-DR+ and CD3+ CD25+ cells. These results indicated that serum IL-1 beta in RA patients reflects the activity of RA, while the serum IFN-gamma and TNF alpha in RA patients may be related to circulating activated lymphocytes and monocytes, respectively.