Serratia marcescens 2170 derivative strains harbouring either the haemolytic plasmid pANN202-312 or pANN202-312R, were able to produce extracellular haemolysin to a greater extent than the corresponding Escherichia coli 5K strains. The haemolysin recovered from S. marcescens cultures supernatants had a molecular weight of 110 kD, and its extracellular production was maximal near the end of the exponential growth phase. The hlyR sequence, which belongs to the haemolytic determinant located in the plasmid pHly152 and enhances haemolysin expression and secretion in E. coli, appeared not to play an equivalent role in S. marcescens.