Clinically relevant Streptococcus spp. were tested for their susceptibility towards human serum transferrin (TR) and lactoferrin (LF). Neither clinical isolates or type strains were inhibited by transferrins (5 mg/ml). All species tested were shown to be able to grow under iron-limiting conditions (< 0.1 microM) and this might account for the lack of TR or LF activity towards streptococci. Even if not sensitive to LF and TR, some species were shown to bind LF in the apo-form.