The effect of sublethal concentrations of£ pephis water suspension and praziquantel water solution on the defense system of Biomphalaria cdexandrina was examined by laboratory experiments.The mean number of haemocytes decreased insignificantly after the first week of snail exposure to the plant extract- Thereafter, a significant increase occurred after the second week of exposure. Treatment with praziquantel showed a significant increase in haemocytes after the 3rd and 4lh weeks of exposure. Haemocytosis represents a response to external stress or certain stimuli. The haemocytes showed behavioral as well as structural changes after long exposure to the tested mollusclcides. The cells had many cytoplasmic inclusions and produce many long spike-like pseudopodia. Haemocytes exhibited an enhanced tendency to aggregate in clusters. Such alterations may be indicative of a state of haemocytes activation. The total plasma protein concentration after treatment with E. pephis and Praziquantel increased plasma than in control snails while no changes in protein concentration of the digestive gland extract. This may be a result of an imbalance in the rate of protein synthesis and the rate of its degradation.The treatment with E peplus and Praziquantel caused a marked increase in acid phosphatase after the 2nd and 4lh weeks. On the other hand, alkaline phosphatase activity increased after the 4th week. This could be explained as defensive and lysosomal activity of the haemocytes. After treatment with the tested substances the plasma and digestive gland extract were capable to agglutinate sheep red