Phage holins and endolysins have long been known to play key roles in lysis of the host cell, disrupting the cytoplasmic membrane and peptidoglycan (PG) layer, respectively. For phages of Gram-negative hosts, a third class of proteins, the spanins, are involved in disrupting the outer membrane (OM). Rz and Rz1, the components of the lambda spanin, are, respectively, a class II inner membrane protein and an OM lipoprotein, are thought to span the entire periplasm by virtue of C-terminal interactions of their soluble domains. Here, the periplasmic domains of Rz and Rz1 have been purified and shown to form dimeric and monomeric species, respectively, in solution. Circular dichroism analysis indicates that Rz has significant alpha-helical character, but much less than predicted, whereas Rz1, which is 25% proline, is unstructured. Mixture of the two proteins leads to complex formation and an increase in secondary structure, especially alpha-helical content. Moreover, transmission electron-microscopy reveals that Rz-Rz1 complexes form large rod-shaped structures which, although heterogeneous, exhibit periodicities that may reflect coiled-coil bundling as well as a long dimension that matches the width of the periplasm. A model is proposed suggesting that the formation of such bundles depends on the removal of the PG and underlies the Rz-Rz1 dependent disruption of the OM.