Pseudomonas exotoxin (PE) is composed of structural domains I, II, and III; when interacting with mammalian cells the function of domain I is cell recognition, the function of domain II is membrane translocation, and domain III functions in ADP ribosylation. PE is secreted by Pseudomonas aeruginosa into its growth medium. The domain responsible for secretion has been examined by expressing modified PE genes in Escherichia coli under the control of a T7 promoter. Without a signal sequence, PE accumulates within the cell, but PE is secreted into the periplasm when part or all of domain I is removed. PE appears in the periplasm and medium when domain I and part of domain II are removed. Domain II alone is secreted into the periplasm, whereas domain III alone remains within the cell. Addition of an OmpA signal sequence results in secretion of mature PE into the periplasm and secretion of domains II-III into the medium. A protein composed of transforming growth factor alpha fused to the amino terminus of domains II-III is secreted into the periplasm without a signal sequence and into the medium with a signal sequence. A protein composed of domain(s) II or II-III fused to the amino terminus of alkaline phosphatase is secreted into the periplasm and the medium with or without a signal sequence. We conclude that domain II contains important information for protein secretion.