Two natural exposure herd studies were conducted at separate geographic locations to evaluate efficacy of a postmilking teat disinfectant that contained 0.25% available iodine. One study was conducted for 11 months in a commercial dairy with a high prevalence of contagious mastitis pathogens. Postdipping was compared with a negative control using a split-herd experimental design. The other study was conducted for 12 months in a research herd with a high prevalence of environmental mastitis pathogens. Postdipping was compared with a negative control using a split-udder experimental design. New intramammary infections by Staphylococcus aureus and Streptococcus agalactiae were reduced significantly in quarters with teats dipped in iodine compared to undipped teats in the herd with a high prevalence of contagious mastitis pathogens. However, the experimental teat dip was not effective against environmental pathogens in either herd. New infections by minor mastitis pathogens (coagulase-negative Staphylococcus species and Corynebacterium bovis ) were significantly lower in quarters of dipped teats compared to undipped teats in both herds. No irritation or chapping of teats was detected and no adverse effects were observed. These data suggest that the low concentration iodine teat dip is effective against contagious pathogens and minor mastitis pathogens, but ineffective against environmental mastitis pathogens.