F1 antigen (Caf1) of Yersinia pestis is assembled via the Caf1M chaperone/Caf1A usher pathway. We investigated the ability of this assembly system to facilitate secretion of full-length heterologous proteins fused to the Caf1 subunit in Escherichia coli. Despite correct processing of a chimeric protein composed of a modified Caf1 signal peptide, mature human interleukin-1 (hIL-1), and mature Caf1, the processed product (hIL-1:Caf1) remained insoluble. Coexpression of this chimera with a functional Caf1M chaperone led to the accumulation of soluble hIL-1:Caf1 in the periplasm. Soluble hIL-1:Caf1 reacted with monoclonal antibodies directed against structural epitopes of hIL-1. The results indicate that Caf1M-induced release of hIL-1:Caf1 from the inner membrane promotes folding of the hIL-1 domain. Similar results were obtained with the fusion of Caf1 to hIL-1 receptor antagonist or to human granulocyte-macrophage colony-stimulating factor. Following coexpression of the hIL-1:Caf1 precursor with both the Caf1M chaperone and Caf1A outer membrane protein, hIL-1:Caf1 could be detected on the cell surface of E. coli. These results demonstrate for the first time the potential application of the chaperone/usher secretion pathway in the transport of subunits with large heterogeneous N-terminal fusions. This represents a novel means for the delivery of correctly folded heterologous proteins to the periplasm and cell surface as either polymers or cleavable monomeric domains.