Leukotoxins produced by staphylococci, especially Staphylococcus aureu
s, have long been considered important virulence determinants. The pre
sent assay examined 26 strains of staphylococci isolated from sheep fo
r their ability to produce factors leukotoxic to polymorphonuclear leu
kocytes deriving from ovine mammary glands. Twenty one strains were co
agulase-negative staphylococci and five coagulase-positive. Of the coa
gulase-negative staphylococci 17 were isolated from subclinical mastit
is and four from the teat skin. The coagulase-positive strains were is
olated from clinical mastitis and caused death to 91-100% of the cells
. The coagulase-negative strains isolated from subclinical mastitis ca
used death to less than 50% of cells, while those from the skin did no
t affect any cells. The concentration of active leukotoxins is an impo
rtant staphylococcal virulence factor. Low production of leukotoxins o
r no production by coagulase-negative staphylococci may determine thei
r ability to survive the mammary gland defenses or cause clinical mast
itis.