Abstract Streptococcus agalactiae is a major neonatal pathogen. Disease progression is characterised by bacterial adaptation from commensal maternal vaginal colonisation to environments associated with neonatal disease, including exposure to blood. To explore this adaptation in vitro, we have used proteomics to identify proteins differentially expressed following growth on Todd Hewitt agar in the presence or absence of 10% v/v human serum. Twelve differentially expressed proteins were identified. Notably, the C protein β antigen and arginine deiminase proteins were upregulated following growth in the presence of human serum, consistent with previous studies implicating these two proteins in the pathogenesis of S. agalactiae disease.