Certain E. coli K12 strains are able to secrete a plasmid encoded 107 K protein into the culture medium. During exponential growth of the cells this protein represents approximately 1% of total cell protein. The presence of the 107 K polypeptide was demonstrated through the fortuitous use of strain MC4100. This gave a largely protein-free culture supernatant, presumably due to minimal lysis of whole cells. Pulse-labelling experiments showed that the secretion of the 107 K polypeptide reached a maximum during the stationary phase of growth, where it represented substantially more than 1% of total cell protein. The 107 K polypeptide is coded by the haemolytic plasmid pHly167, and appears to be related to a previously reported intracellular "precursor" form of the alpha-haemolysin (Goebel and Hedgpeth 1982). However, additional extracellular factors appear to be required for alpha-haemolysin activity since several nonhaemolytic mutants still secrete this protein.