The soluble interleukin-2 receptor (SIL-2R) levels in 46 patients with primary nephrotic syndrome were examined. The values in the nephrotic patients exceeded those in healthy controls (p less than 0.05), and were the same at the nephrotic and non-nephrotic stages. The serum interleukin-2 (IL-2) levels in the nephrotic patients were low and did not correlate with the serum SIL-2R levels. Only at the nephrotic stage were the latter closely correlated with the serum IgG (p less than 0.001) but not with the serum IgA or IgM. The SIL-2R production by phytohemagglutinin stimulated peripheral blood lymphocytes (PBL) in the nephrotic patients was less at the nephrotic and non-nephrotic stages than that in the healthy controls. The IgG production by pokeweed mitogen stimulated PBL was low at the nephrotic stage but not at the non-nephrotic stage. Based on these findings, it is suggested that SIL-2R determinations should prove useful in clarifying the relationship between cellular and humoral immunological disorders in nephrotic syndrome.