The bacteriostatic activity of bovine lactoferrin (Lf) against mastitic bacteria was assessed with an in vitro microassay. The most susceptible species was Escherichia coli; all of the 35 isolates tested were susceptible to bacteriostasis by apo-Lf (0.1 mg ml-1), although a few strains showed a lower degree of inhibition. Heterogeneity among strains was more pronounced among 10 isolates of Staphylococcus aureus, four of which were apparently unaffected by apo-Lf (1 mg ml-1). Under the same conditions, Streptococcus agalactiae (six isolates) and Str. uberis (five isolates) resisted the bacteriostatic action of apo-Lf.