Annexin II (AII) is a member of a family of glycoproteins which bind negatively charged phospholipids in a calcium-dependent manner. Annexins are membrane-associated proteins, expressed both in normal and malignant cells, but have also been detected as soluble molecules in serum and other body fluids. Because of their adhesive properties, it has been suggested that annexins play a role in the metastatic process. An ELISA was established for quantification of soluble AII. Within-run variation was 5.2-10.4% and run-to-run variation 12.4-15.6%. Soluble AII was detected in all sera studies. A strongly positive serum was arbitrarily given the value 100 AII units and used as reference serum. The mean level in sera from 20 normal blood donors was 49 (SE 5.6) AII units. Sera from peripheral blood of five patients with renal cell carcinoma and sera from blood obtained from the renal vein of the same patients contained 47 (SE 20) and 83 (SE 28) AII units, respectively. In two patients, AII levels were increased in renal vein serum as compared with peripheral blood serum. Interestingly, in both cases, and in none of the three remaining cases, phytohaemagglutinin-stimulated lymphoproliferation was suppressed by renal vein serum as compared with peripheral blood serum. Affinity absorption of AII from the renal vein sera with increased AII levels strongly reduced their immunosuppressive activity. Addition of affinity-purified AII to cell cultures suppressed lymphoproliferation. These data show that the level of AII is markedly increased in renal vein sera from some patients with renal cell carcinoma, suggesting that AII may be locally released in vivo. The study also demonstrates an immunosuppressive effect of soluble AII in vitro. We speculate that soluble AII released by the tumour has immunosuppressive properties. This study identifies soluble AII as a novel immunosuppressive factor in sera from patients with renal cell carcinoma. A further study including a larger number of patients is currently in progress, in order to investigate the pathological significance of this finding.