In order to determine the changes in serum and blood factors of rainbow trout following experimental infection with a pathogenic Aeromonas hydrophila, the cause of haemorrhagic septicemia in fish, three groups of rainbow trout (10 fish in each group) were injected intraperitoneally with 105, 106 and 107 of this bacteria respectively. The same number of fish in a control group were injected similarly with physiologic saline. After the appearance of clinical symptoms of haemorrhagic septicemia in fish, samples from skin mucus and blood of the fish were collected in order to determine lysozyme, anti- Aeromonas hydrophila immunoglobulins (measured by plate method and ELISA respectively), total white and red blood cells( WBC/RBC). Results showed that the lysozyme level in skin mucus and sera was significantely higher than the control group (p<0.05) while the immunoglobulin level of skin mucus of experimentally challenged fish was not significantely higher than that of control group except that of the group 2 and 3 of the experimental fish. WBC/RBC and hematocrit of the experimentally challenged rainbow trout were significantely higher than the control. Thus infection of rainbow trout with Aeromonas hydrophila increases the level of mucus and serum lysozyme, serum immunoglobulins, decreases RBC count and heamatocrit percentage.