Interleukin-1 (IL-1), tumor necrosis factor (TNF), and interferon-_γ (IFN_γ) inhibit insulin release and may be cytotoxic to isolated rodent pancreatic islets. In this study we examined the effects of IL-1, TNF, and IFN_γ on the viability and hormone secretion of islets isolated from adult human pancreas and maintained in monolayer culture. IL-1 and TNF were cytotoxic to the islet cells (20-30% cell lysis) in a ⁵¹Cr release cytotoxicity assay, and IFN_γ had only small effects (<10% lysis). Combination of maximally cytotoxic concentrations of IL-1 (10 U/mL) and TNF (10³ U/mL) produced an additive cytotoxic effect. IFN_γ (10³ U/mL) acted synergistically with IL-1 and TNF, and the three cytokines added together produced maximal islet cell lysis (46.4 ± 4.3%). Assay of insulin and glucagon in the islet monolayers revealed that IL-1, TNF, and IFN_γ inhibited both B- and A-cell secretory functions; however, only IL-1 and TNF produced permanent decreases in insulin and glucagon contents in the islet cultures. These findings indicate that IL-1 and TNF, as single agents, are cytotoxic to human islet cells, and that this cytotoxicity can be amplified by combining the cytokines and/or adding IFN_γ. However, the lack of specificity for B-cells in vitro suggests that additional factors might be operative in vivo for the cytokine products of macrophages and lymphocytes infiltrating islets to produce the B-cell-specific damage characteristic of type 1 diabetes.