Tris(hydroxymethyl)aminomethane (tris) inactivates Lactobacillus lactis bacteriophages LL-H in vitro. The inactivation is caused by DNA ejection. This effect occurs in tris-HCl buffer only. Both pH and temperature affect the sensitivity of the phage to the tris treatment. The divalent cation Mg2+ prevented the inactivating effected of tris at concentrations about 10(3)-fold lower than monovalent cation K+.