The construction of live recombinant bacterial vaccines requires a reasonably sophisticated genetic system for the introduction, stabilization and expression of foreign antigen genes. Bacteriophages offer a rich collection of tools that can be used for vaccine construction, including site-specific integration-proficient vectors, non-antibiotic selectable markers and signals for efficient transcription and translation of foreign genes. We describe the characterization of a temperate phage of the mycobacteria, mycobacteriophage L5, and application of these phage studies for the construction of recombinant BCG vaccines.