Following growth in a chemically defined medium (CDM), five strains of Streptococcus uberis were tested for their ability to survive killing by bovine neutrophils. Strains 0140J, ST10, EF20 and C221 were easily killed, whereas strain C197C was highly resistant. The ability of strain 0140J to resist phagocytosis and killing was increased by supplementation of the growth medium with milk whey, casaminoacids, casein, or, to a lesser extent, bovine serum. Supplementation of the growth medium with yeast extract or bovine serum albumin did not affect the resistance of this strain. Following growth in CDM supplemented with casein, strains ST10 and C221, like strain 0140J, were significantly more resistant to killing by neutrophils. The resistance of strains EF20 and C197C was unaffected by the addition of casein to the medium; strain EF20 remained susceptible and strain C197C highly resistant to killing. The effect of supplementing the growth media with components other than casein was only studied for strain 0140J. Decapsulation of strains C197C, ST10 and 0140J, grown in CDM + casein, with type-X hyaluronidase did not significantly affect their ability to survive in the presence of bovine neutrophils.