Although phage lambda represents a well studied biological systems, it has certain features that remain obscure. Among these is the function of the roughly one third of the phage genome dispensable for growth in the laboratory, yet retained despite undoubted pressure to economize. Here we report that these 'accessory' sequences contain two genes which are expressed during lysogeny, and encode host-cell envelope proteins. One of these is lom, the product of which is found in the bacterial outer membrane, and is homologous to virulence proteins of two other enterobacterial genera. The other gene, previously unidentified, we designate bor. Expression of bor significantly increases the survival of the Escherichia coli host cell in animal serum. This property is a well known bacterial virulence determinant--indeed, bor and its adjacent sequences are highly homologous to the iss serum resistance locus of the plasmid ColV2-K94, which confers virulence in animals. These results show that the lambda prophage is more transcriptionally active than has long been assumed, and suggest that lysogeny may generally have a role in bacterial survival in animal hosts, and perhaps in pathogenesis.