In Lymnaea stagnalis injections with dead Escherichia coli or Staphylococcus saprophyticus bacteria resulted in an enhanced clearance of both live S. saprophyticus and E. coli injected 4 days later. A non-specific activation of the internal defence system was concluded from these findings. The activation was dose-dependent: pre-injections with high doses resulted in a higher increase in the clearance capacity of the snails than pre-injections with low doses of bacteria. The state of increased activity of the defence system, induced with injection of dead E. coli, lasted at least 64 days. The heightened responses of the defence system were probably due to an activation of the blood cells (amoebocytes) since: 1) amoebocyte numbers increased faster in bacteria pretreated snails than in control animals; 2) ultrastructural observations revealed that the amoebocytes of bacteria pre-treated animals had a more ruffled outline than those of control snails; 3) amoebocytes from sensitized snails showed a higher phagocytic activity in vitro; 4) mitotic activity of amoebocytes increased after snails had been injected with bacteria.