During the course of an acute, fatal mastitis in mice caused by intra-mammary injection of ∼ 105 cfu of Staphylococcus aureus, small numbers of staphylococci were detected in liver, spleen, kidney and heart. Transport from the mammary gland was by the venous blood system with no evidence of lymph node involvement. Intravenous injection of 100 times more staphylococci (∼107 cfu) than were fatal by the intramammary route failed to kill the mice and small numbers of staphylococci were found in the visceral organs. However, intravenous injection of ∼ 108 cfu of Staph. aureus was fatal and large numbers of staphylococci were found in the visceral organs. Treatment with cloxacillin (500 μg) by the intramammary or subcutaneous routes, or equally divided between these two routes, cleared the visceral organs of staphylococci; subcutaneous therapy had a limited and variable effect on numbers of staphylococci in the mammary gland while intramammary and divided treatments significantly reduced the numbers of staphylococci. Thus, staphylococci are disseminated to visceral organs during acute staphylococcal mastitis, but this makes a negligible contribution to the clinical disease.