In this study, the protection level of a commercial bivalent vaccine, developed with the aim of boosting the resistance of cultured sea bass against bacterial diseases, including vibriosis and pasteurellosis, was determined during a one-year period. The antibody level, which represents a significant specific immune system parameter developing in fish samples vaccinated by injection against Vibrio anguillarum serotype 01 and Photobacterium damselae subspecies piscicida, was identified by utilizing different serological methods. During the study period of one year, 5 sampling studies were carried out in a commercial sea bass farm, which is situated in the Aegean Region of Turkey. In the mentioned sampling studies, the collection of blood and serum samples was performed from a total of 80 vaccinated and 20 non-vaccinated fish samples with a weight ranging between 10-100 gr that were reared in floating cages. According to the findings of agglutination tests and Enzyme-Linked Immunosorbent Assay (ELISA), it was determined that the protective level of the commercial vaccine in question utilized in sea bass in field trials was higher against V. anguillarum compared to P. damselae subsp. piscicida subspecies piscicida. Because of observing an excessively high antibody level against V. anguillarum 120 days following the injection, a high protective level was found in the mentioned period. At the same time, a comparatively low antibody level developed against P. damselae subsp. piscicida, and, thus, a low protective level against the above-mentioned bacterium was identified. In the present research, the long-term protective impact of bivalent vaccines was firstly investigated serologically.