Abstract  Aggregatibacter aphrophilus, a Gram negative bacterium, found in the oral cavity, causing cerebral abscesses and infective endocarditis, has been shown to be serum resistant in previous studies. Bacterial secretion systems are important for bacteria as they transfer virulence factors into other bacteria or host cells as an attack. A. aphrophilus encodes a type VI secretion system, which is a spike-like membrane protein, mainly consisting of a hemolysin co-regulated protein (Hcp). In this work, it was tested if Hcp would contribute to serum resistance of A. aphrophilus. Firstly, to assess Hcp contribution to serum resistance, a bacterial serum killing assay-method was used and data was collected from three independent experiments. Two strains of A. aphrophilus were used in the experiments: the laboratory strain HK83 and a HK83 hcp mutant strain. The results showed that Hcp provided no significant effect on serum resistance of A. aphrophilus. Secondly, optical density measurements were made for growth curve analysis, to determine if the HK83 hcp mutant strain had an impact in growth compared to HK83. The results indicated that the HK83 hcp mutant strain had a somewhat reduced growth compared to its parental strain.