Results of bacteriological culture of 5426 pairs of duplicate quarter milk samples collected were analyzed for agreement. Overall, the percentage of agreeing pairs was 98.1%. The percentage agreement by infection type was greater for the contagious pathogens, Streptococcus agalactiae (96.4%) and Staphylococcus aureus (94.2%), than for other Streptococcus spp. (81.6%) and coliform organisms (55.6%). Single quarter samples may be adequate for determining status of quarter infection with Strep. agalactiae and Staph. aureus.