The Bam machine assembles β-barrel membrane proteins into the outer membranes of Gram-negative bacteria. The central component of the Bam complex, BamA, is a β-barrel that is conserved in prokaryotes and eukaryotes. We have previously reported an in vitro assay for studying the assembly of β-barrel proteins by the Bam complex and now apply this assay to identify the specific components that are required for BamA assembly. We establish that BamB and BamD, two lipoprotein components of the complex, bind to the unfolded BamA substrate and are sufficient to accelerate its assembly into the membrane.