['We previously reported significant variations in oxidation status and molecular length among sources and lots of human serum albumin (HSA) commercial preparations intended for clinical use. In this report, we investigated what effect the presence of HSA products have on the immune response in vitro.', 'Laboratory study.', 'Trauma research basic science laboratory.', 'Activated human peripheral blood mononuclear cells.', 'Six commercial HSA preparations were tested for their effect on cytokine release from activated human peripheral blood mononuclear cells (PBMCs) and T-lymphocytes. Mass spectrometry analysis of aspartyl-alanyl diketopiperazine (DA-DKP) content of HSA and percentage of HSA having lost its amino terminal dipeptide aspartyl alanyl (HSA-DA) were correlated.', 'Human PBMCs were cultured in the presence of six commercial HSA preparations and activated via the T-cell receptor complex. A cloned T-lymphocyte cell line, activated with specific antigen, was also cultured with both synthetic DA-DKP and small molecular weight extracts from the commercial HSA tested. Supernatants were quantified by enzyme-linked immunosorbent assay for interferon-gamma and tumor necrosis factor-alpha content. DA-DKP was extracted from HSA by centrifugal filters and quantified by anion exchange liquid chromatography coupled to negative electrospray ionization mass spectrometry. HSA species were determined by reverse phase liquid chromatography coupled to positive electrospray ionization, time of flight mass spectrometry. All HSA preparations significantly inhibited the in vitro production of interferon-gamma and tumor necrosis factor-alpha by activated PBMCs. DA-DKP was detected in all HSA sources at concentrations ranging between 42.0 and 79.6 microM. A synthetic form of DA-DKP possessed similar immunosuppressive qualities in a dose-dependent manner on T lymphocytes.'] Are commercial human albumin preparations for clinical use immunosuppressive in vitro?