Viridans streptococci are a heterogeneous group of Gram-positive bacteria that are normal inhabitants of the mouth, upper gastrointestinal tract and oropharynx. These organisms are typically thought of as of low virulence, classically as the cause of infective endocarditis, although recently they have been implicated in serious infections in other settings. In particular, viridans group streptococci have been described as responsible for the alpha-streptococcal shock syndrome in neutropenic patients. The mechanism by which viridans streptococci cause bacteraemia associated with adult respiratory distress syndrome (ARDS) in these patients has not been elucidated. Using enzyme-linked immunosorbent assays, we compared the ability of cell-free bacterial supernatants derived from commensal and clinical strains of viridans streptococci to induce the pro-inflammatory cytokines tumour necrosis factor alpha (TNF-alpha), tumour necrosis factor beta (TNF-beta) and interleukin 8 (IL-8) from human peripheral blood mononuclear cells (PBMC) in vitro. Supernatants of clinical isolates induced significantly more TNF-beta (P < 0.002) and IL-8 (P < 0.001) than did supernatants from commensal strains. The increased production of IL-8 by the clinical strains may be of importance in view of the role of IL-8 in the pathogenesis of the acute respiratory distress syndrome (ARDS), one of the principal clinical features of the alpha-streptococcal shock syndrome.