In the spiny lobster (Panulirus interruptus), unlike other crustaceans most of the prophenoloxidase (proPO) was detected in cell-free plasma (86.3%). In spite of its location, lobster proPO activating system has a similar activation mechanism to other crustacean proPO systems. Haemocyte lysate was able to activate the plasma proPO indicating location of the prophenoloxidase activating enzyme (PPAE) in haemocytes. Lobster haemocyte PPAE was isolated by affinity chromatography and its participation as activating enzyme was demonstrated. This enzyme is a serine-proteinase that transforms the inactive form (proPO) to an active one (phenoloxidase). The PPAE was also present in the cell-free supernatant of haemocytes previously incubated with Vibrio alginolyticus.