Filamentous phage proteins pI and pIV are morphogenetic proteins required for phage assembly but not part of the virion. Neither pI nor pIV from the related phages f1 and IKe can substitute for its equivalent in the other phage. When the two proteins are supplied as pairs, however, partial restoration of heterologous phage assembly occurs. This observation strongly suggests that the two proteins interact. A selection for revertants of a temperature sensitive mutant of f1 gene IV resulted in the isolation of a suppressor mutation in gene I. This suppressor is allele specific, and thus supports the hypothesis that pI and pIV interact. A selection for IKe phage that can efficiently utilize paired pI and pIV from from f1 led to the isolation of a phage with a mutation in gene VIII, which encodes the major coat protein of the virus. Analysis of the system suggests that it is pI that interacts with both pIV and pVIII. Thus the process by which filamentous phage are concomitantly assembled and secreted across the cell membranes is likely to involve a series of protein-protein interactions that are accessible to genetic analysis.