Abstract.Extracellular hacmolytic activities ofAeromonas salmonicidassp.salmonicidato salmon red blood cells were shown to be due to different forms of the membrane‐active enzyme glyccrophospholipidrcholcstcrol acyltransferase (GCAT). About 10% of the total haemolytic activity was due to a high molecular mass complex of LPS and GCAT (mol. mass>1000kDa), containing 35–50% neutral sugars and 1.5–2.0% protein. Some haemolytic activity (30–40% of total), corresponding to 50–70kDa by gel filtration, also contained GCAT‐activity and may represent aggregated forms of GCAT. However, about 50% or more of the haemolytie activity was due to a protein of 26kDa free GCAT. Rabbit antibodies to GCAT neutralized the hacmolytic activity of both GCAT and GCAT‐LPS. A transposon‐produccd serinc protease negative mutant of the sameA. salmonicidastrain showed reduced haemolytic activity. The mutant produced a 38‐kDa GCAT proform of low hacmolytic activity. The proform was processed by autogenous scrinc protease to a highly hacmolytic 26‐kDa molecule with pl 6.3, similar to GCAT of the parent strain. The weakly haemolytic GCAT‐LPS analogue of the mutant strain did not contain detectable amounts of the 26‐kDa molecule and was not