We have investigated the influence of kidney function on soluble interleukin-2 receptor (S-IL-2R) serum levels. S-IL-2R is a glycoprotein with a molecular weight of 45 kDa. The serum concentration of S-IL-2R was positively correlated to serum creatinine (r =0.35) and complement factor D (r = 0.30), a low molecular weight protein, and the reciprocal value of S-IL-2R was negatively correlated to 51Cr-EDTA clearance (r= -0.35). When measuring S-IL-2R and factor D in a wide range of serum creatinine (140-1,380 (xmol/l), a high positive correlation was found between factor D and serum creatinine. S-IL-2R and serum creatinine had r = 0.92 and r = 0.79, p = 0.0001, respectively. The correlation coefficient between factor D and S-IL-2R was 0.86. S-IL-2R is used as a marker for an increased activity of the immune system and in a variety of lymphoproliferative disorders. However, it is important to keep in mind that a reduced kidney function may contribute to the increased S-IL-2R levels seen in different immune diseases. Therefore, renal function should be taken into account in the interpretation of elevated plasma concentrations of S-IL-2R.