Enterococcus faecalis is a rare but recognized cause of prosthetic joint infection. It is notorious for formation of biofilm on prosthetic surfaces. We hypothesized that a ‘serum factor’ was responsible for transformation of E. faecalis from its planktonic form to a biofilm existence upon making contact with prostheses. Using a novel ‘proteomic approach’, we studied the protein expression profiles of this bacterium when grown on an artificial surface in a serum environment against a control. E.faecalis 628 transconjugant formed by conjugation clinical strain (E55) and laboratory strain (JH2-2) was used to inoculate each of rabbit serum (RS) and Brain Heart Infusion (BHI) agar as a control and grown for 24 hours. Proteins were harvested for analysis in fractions including cell surface, membrane and cytosolic proteins. Recovered proteins were separated using 2-dimentional polyacrylamide gel electrophoresis (2D PAGE). Gels were stained and spots of interest harvested. These were analyzed using MALDI mass spectrometry followed by peptide mass fingerprinting using online database searches. Two surface exclusion proteins Sea1 and PrgA were only expressed from the serum culture. These proteins are both encoded by genes very close to the gene for enterococcal aggregation substance PrgB, which plays an integral role in biofilm formation. PrgA and PrgB are both encoded by the prgQ operon and hence expressed simultaneously upon activation of the operon. This tendency for serum only protein expression suggests the possibility of a pheromone-like activator in serum that could be a potential therapeutic target for management of biofilm associated E. faecalis prosthetic infections