The effect of activated platelets on cytokine production by human peripheral blood mononuclear cells (PBMC) was investigated. When PBMC were coincubated with activated autologous platelets amid lipopolysaccharide (LPS, 50-100 pg/mL) for 8 h, the production of interleukin (IL)-1alpha increased 11- to 18-fold and tumor necrosis factor (TNF)-alpha 3- to 5-fold compared with PBMC without platelets. Activated platelets in a dual-chamber well that prevented platelet-PBMC contact but permitted passage of soluble factors enhanced IL-1alpha production (P < .01). Platelet-PBMC contact in the chamber resulted in a further enhancement of IL-1alpha production. These data suggest that platelet-PBMC interaction, both directly and with platelet-derived factors, enhances production of shock-producing IL-1alpha and TNF-alpha, albeit differently. The interaction of platelets with monocytes may play an important role in the pathophysiology of sepsis and disseminated intravascular coagulation.