Secoiridoides (oleuropein and derivatives), one of the major classes of polyphenol contained in olives and olive oil, have recently been shown to inhibit or delay the rate of growth of a range of bacteria and microfungi but there are no data in the literature concerning the possible employment of these secoiridoides as antimicrobial agents against pathogenic bacteria in man. In this study five ATCC standard bacterial strains (Haemophilus influenzae ATCC 9006, Moraxella catarrhalis ATCC 8176, Salmonella typhi ATCC 6539, Vibrio parahaemolyticus ATCC 17802 and Staphylococcus aureus ATCC 25923) and 44 fresh clinical isolates (Haemophilus influenzae, eight strains, Moraxella catarrhalis, six strains, Salmonella species, 15 strains, Vibrio cholerae, one strain, Vibrio alginolyticus, two strains, Vibrio parahaemolyticus, one strain, Staphylococcus aureus, five penicillin-susceptible strains and six penicillin-resistant strains), causal agents of intestinal or respiratory tract infections in man, were tested for in-vitro susceptibility to two olive (Olea europaea) secoiridoides, oleuropein (the bitter principle of olives) and hydroxytyrosol (derived from oleuropein by enzymatic hydrolysis and responsible for the high stability of olive oil). The minimum inhibitory concentrations (MICs) calculated in our study are evidence of the broad antimicrobial activity of hydroxytyrosol against these bacterial strains (MIC values between 0.24 and 7.85 microg mL(-1) for ATCC strains and between 0.97 and 31.25 microg mL(-1) for clinically isolated strains). Furthermore oleuropein also inhibited (although to a much lesser extent) the growth of several bacterial strains (MIC values between 62.5 and 500 microg mL(-1) for ATCC strains and between 31.25 and 250 microg mL(-1) for clinical isolates); oleuropein was ineffective against Haemophilus influenzae and Moraxella catarrhalis. These data indicate that in addition to the potential employment of its active principles as food additives or in integrated pest-management programs, Olea europaea can be considered a potential source of promising antimicrobial agents for treatment of intestinal or respiratory tract infections in man.