2535 Background: A decrease in the T cell receptor ζ chain expression has been described in malignancy and chronic infections. The association with response to treatment and the mechanisms leading to this diminished expression are unclear. In vitro models suggest that arginase production may decrease CD3ζ expression.Methods: We examined the expression of CD3ζ chain and arginase activity in preRx samples of peripheral blood mononuclear cells from pts with mRCC receiving IL-2 Rx on a CWG Phase III trial, ζ chain expression was studied by florescence intensity (MFI) and serum arginase activity by conversion of arginine to ornithine. Results were compared to blood obtained from 33 healthy normal volunteers (NV). Results: Of 106 with a pre-Rx blood samples 18 pts responded (7 CR, 11 PR). MFI for CD3 ζ was 21.39 +/- 8.15 for the pts vs 35.43 +/- 12.24 for the NV. Mean arginase was 128.55 for pts vs 32.63 for NV. Serum arginase levels and CD3 ζ chain expression were inversely related (r= -0.31, p<0.01 ). Low and...