-Experimental and therapeutic use of islet cell preparations could benefit from assays that measure variations in the mass of living -cells. Because processes of cell death can be followed by depletion and/or discharge of cell-specific substances, we examined whether in vitro conditions of -cell death resulted in changes in tissue and medium content of insulin and of ␥-aminobutyric acid (GABA), two -cell-specific compounds with different cellular localization and turnover. Exposure of rat purified -cells to streptozotocin (5 mM, 120 min) or to the nitric oxide donor GEA-3162 (GEA; 50 M, 120 min) caused 80% necrosis within 24 h; at the end of this period, cellular insulin content was not significantly decreased, but cellular GABA content was reduced by 70%; when cultured at basal glucose (6 mM), the toxin-exposed cells did not discharge less insulin but released 80% less GABA in the period 8 -24 h. As in rat -cell purification, GABA comigrated with insulin during human islet cell isolation. Twenty-four hours after GEA (500 M, 120 min), human islet cell preparations exhibited 90% dead cells and a 45 and 90% reduction, respectively, in tissue insulin and GABA content; in the period 9 -24 h, insulin discharge in the medium was not reduced, but GABA release was decreased by 90%. When rat -cells were cultured for 24 h with nontoxic interleukin (IL)-1 concentrations that suppressed glucose-induced insulin release, cellular GABA content was not decreased and GABA release increased by 90% in the period 8 -24 h. These data indicate that a reduction in cellular and medium GABA levels is more sensitive than insulin as a marker for the presence of dead -cells in isolated preparations. Pancreatic GABA content also rapidly decreased after streptozotocin injection and remained unaffected by 12 h of hyperglycemia. At further variance with insulin, GABA release from living -cells depends little on its cellular content but increases with IL-1-induced alterations in -cell phenotype.islet; diabetes; -cell death; ␥-aminobutyric acid PROCESSES OF CELL DEATH can be associated with depletion and/or discharge of cell-specific substances. Cell losses can then be monitored through assays of these compounds in tissue and/or blood. Such method is not yet available to detect destruction of pancreatic -cells during development of diabetes, but release of insulin-mRNA might qualify as a marker for a massive destruction of a -cell graft (22). There is not an established method for quantifying the mass of living -cells in isolated tissue fractions. This lack makes it difficult to assess the therapeutic potential of islet cell preparations and to compose grafts accordingly. Tissue insulin content and secretory activity have been used as quality control tests in islet transplantation (9, 10, 13, 21), but their significance as indexes for the living -cell mass needs to be further validated. Living -cells can indeed markedly vary in their insulin content and can variably degranulate during isolation and culture procedures; furthermor...