Lactobacillus delbrueckii subsp. lactis (Lb. lactis) CNRZ 326 is widely used in the propagation of Lb. delbrueckii bacteriophages. In this study, evidence is presented that this strain possesses a restriction-modification (R/M) system. The mitomycin C-induced temperate bacteriophage 1b539 has a reduced efficiency of plaquing (EOP) on CNRZ 326 cells (EOP = 10(-3)), but after several passages on this strain, or on the indicator strain Lb. lactis LKT, the recovered phages (phages 1b539.326 and 1b539.LKT) have an EOP equal to 1. Restrictive development on CNRZ 326 was also observed after phage 1b539.326 was propagated on the strain Lb. lactis CRL 934. The R/M system was also active against the virulent Lb. delbrueckii phage 11-h. Plasmid DNA was not detected in CNRZ 326, which suggests that the R/M system described is chromosomally encoded.