The bacteriophage-host sensitivity patterns of 16 strains of Lactococcus lactis originally isolated from a mixed strain Cheddar chef se starter culture were determined. Using phages obtained from cheese factory whey, four of the strains were found to be highly phage resistant. One of these isolates, Lactococcus lactis subsp, cremoris HO2, was studied in detail to determine the mechanisms responsible for the phage insensitivity phenotypes, Conjugal transfer of plasmid DNA from strain HO2 allowed a function to be assigned to four of its six plasmids. A 46-kb molecule, designated pCI646, was found to harbor the lactose utilization genes, while this and plasmids of 58 kb (pCI658), 42 kb (pCI642), and 4.5 kb (pCI605) were shown to be responsible for the phage resistance phenotypes observed against the small isometric-headed phage phi 712 (936 phage species) and the prolate-headed phage phi c2 (c2 species). pCI658 was found to mediate an adsorption-blocking mechanism and was also responsible for the fluffy pellet phenotype of cells containing the molecule. pCI642 and pCI605 were both shown to be required for the operation of a restriction-modification system.