Nucleic acid sequence analysis has indicated that the leukotoxin determinant from Pasteurella haemolytica is related to the hemolysin determinant from E. coli. The cloning and expression in E. coli of the lktCA genes has been previously reported, but the existence of leukotoxin secretory genes equivalent to hlyBD has not been documented. In this report we demonstrate that a 4.0 kb segment of P. haemolytica genomic DNA distal to the lktA gene, when expressed in trans to the previous cloned lktCA genes, allow the synthesis and secretion of active leukotoxin from E. coli. Complementation analysis using the cloned hlyB and hlyD genes indicates that this secretory locus derived from P. haemolytica contains two genes which we designate, by analogy, lktB and lktD.