The concentrations of lactoperoxidase (LP) and thiocyanate (SCN-) in the mammary secretions of 4 dairy cows in the early dry period were similar to or higher than concentrations in the milk before drying off. The concentrations of free cystine progressively increased in the secretions beginning 3 to 5 days after the last milking; the mean concentrations were 0.66 mumoles/L before drying off and 6.66 mumoles/L after drying off. The mean concentrations of free cysteine were 0.28 mumoles/L before drying off and 1.4 mumoles/L after drying off. The secretions, when diluted in steamed milk, showed greater stimulation of Streptococcus agalactiae growth as the drying-off period progressed. This increase in stimulatory activity was attributed primarily to the increased concentrations of cystine because cystine counteracts the LP/SCN-/hydrogen peroxide inhibitory system for S agalactiae. This effect on the LP system may account for any increase in susceptibility to S agalactiae under infection during the dry period. In 3 other cows, the mammary secretions on the day of calving had lower mean concentrations of LP, SCN-, and free cystine and cysteine than those obtained 4 to 5 days before, and 7 to 8 days after calving.