N-Chlorotaurine (NCT) is a promising endogenous agent for topical treatment of infections. We tested the tolerability and pharmakokinetics of NCT in the bovine mammary glands in a phase 1 study. Three concentrations of NCT in water (0.1%, 1.0%, 2.0%) were administered intramammarily in each of two cows. Into two quarters of the udder 100 ml NCT was injected into each twice daily for 5 d, while 0.9% NaCl was injected into the other two quarters in a randomized and blinded manner. Samples of milk were taken to determine the number of leucocytes and the activity of NCT, and samples of urine and blood to determine the taurine and chloride concentration. Chloride concentrations in serum samples were determined by an ISE-Unit of a Modular-System of the Roche Diagnostics company. The udder was monitored clinically for signs of inflammation. Oxidative activity could be detected in the milk after single irrigations for 15 min (0.1% NCT) and for maximally 5 h (1% and 2% NCT), respectively. On day 2, leucocytes increased to 4 x 10(6)/ml in the NCT group, while they remained 1 x 10(6)/ml in the saline group. However, on days 3-5 they increased to (5-7) x 10(6) in both the NCT and control group without any statistical difference. One day after the end of dosing the number decreased significantly and reached the baseline (<1 x 10(6)/ml) on day 10. The decrease was similar in both groups. Except for sporadic slight induration of single quarters in both groups and slight reduction of milk performance no disorders occurred. Taurine levels in blood and urine did not change. Irrigation of the bovine mammary gland with both NCT and saline caused a transient increase of leucocytes in the milk, but no severe side effects. The absence of residues and decay products may be a great advantage of NCT over other antimicrobial agents.