The lysA gene specifying an endolysin of Lactobacillus delbrueckii subsp, bulgaricus bacteriophage mvl, was cloned and expressed in Escherichia coli. The 4.05-kb restriction fragment containing this gene was analysed by restriction and deletion mapping, and by subcloning. The nucleotide sequence of a 1150-bp fragment coding for an active lysin was determined. The lysA gene consists of 585 bp and codes for a protein of a deduced Mr of 21 120, which agrees with the size based on in vivo transcription/translation studies. The deduced amino acid sequence of the mvl lysin (LysA) was compared to that of other known lyric enzymes. Significant homology was observed with the N-terminal portion of the muramidase of the fungus Chalaropsis and that of the muramidase of the Streptococcus pneumoniae phage Cp-l, suggesting that LysA might be a muramidase. In E. coil, the cloned lysA gene was able to complement the muramidase-defective bacteriophage ~.Ram5, proving that the products of these two genes are interchangeable. The iysA gene is preceded by an open reading frame with unknown function and no characteristic prokaryotic promoter sequences could be detected upstream from lysA, suggesting that this gene is part of an operon.