Strains of Staphylococcus aureus were isolated from bovine mastitis, subcultured and maintained in the laboratory for up to 3 years. Encapsulation was assessed by production of a diffuse colony in serum-soft agar. Eight (4%) of 200 strains were encapsulated. Three rapid passages of the remaining 192 strains through either brain-heart infusion broth containing 30% serum or modified 110 medium retrieved the capsule in 75%, but this was rapidly lost after subculture on blood agar. The stimulation of capsule production was studied in 18 of these strains by addition of various components to the passaging medium. Heat-labile factors in serum, milk and mastitic milk enhanced capsule production while bovine serum albumin, an extract of polymorphonuclear leucocytes, NaCl and immunoglobulins had minimal effect. The results indicate that encapsulation is common in bovine staphylococci and while it is lost on subculture, may be retrieved under appropriate conditions.