T: Washington State University dairy milks 135 to

145 cows twice daily in a double-5 herringbone parlor. Prior to 1993, the percentage of cows with intramammary infections (IMI) caused by Staphyl-ococcus aureus was< 3%, and the mean somatic cell count (SCC) for the herd was< 200,000 cells/ml. The hygiene program at the time of milking included preand postmilking teat disinfection with a 0.1% iodine solution," single-service cloth towels to clean and dry udders, rubber gloves worn by the milkers, and an iodine solution back flush system for the milking unit. This hygiene program, along with antibiotic treatment of nonlactating cows, had been proven effective in the control of S aureus mastitis." An increase in mean SCC for the herd was noticed in late autumn 1993 (Fig 1). Milk samples were collected aseptically from all lactating cows for bacteriologic culture to identify pathogens associated with the increase." The results of this initial culture suggested that 24 cows had IMI caused by S aureus. Given the rapid increase in SCC and preliminary culture results, we hypothesized that l strain of S aureus had been introduced into the herd and had become widely dis-seminated. The investigation to test our hypothesis began in January 1994. Milk samples were aseptically collected" from each lactating cow at monthly intervals for 18 months. Milk from each mammary quarter was collected into a separate vial for quarter samples or combined into l tube for composite samples. Samples were stored frozen at–20 C (–4 F) until cultures were performed. Extensive environmental sam-pling was also performed. Samples from milkers latex gloves (n= 62), milkers' nares (8), and teat dip cups (32), were taken, using sterile cotton-tipped swabs moistened in sterile Vogel and Johnson (VJ) liquid medium." Samples from milking-unit liners before (n