Background: In neonatal chylothorax, thoracic lymphatic drainage is ineffective. The resultant effusions often require drainage, leading to a loss of immune components. Affected infants can be managed with formula or defatted human milk feedings low in long chain triglycerides to decrease lymph production. We hypothesized that there is no significant difference in the immunological profile or antibacterial effect of full fat and defatted human milk. Methods: Milk from lactating mothers was divided into one aliquot that was defatted via centrifugation with the full fat aliquot as control. Macronutrient content was analyzed with mid-infrared spectroscopy. Flow cytometry was used to measure immune cell populations. Lactoferrin, lysozyme, IgA and IgG values were determined using ELISA. The antibacterial properties were determined by inoculating paired full fat and defatted milk samples with Escherichia coli or Streptococcus pneumoniae bacteria and performing colony counts. Results: Compared to full fat milk, defatted milk demonstrated decreased total energy and fat and increased carbohydrate concentrations. Defatted milk demonstrated a significant decrease in all immune cell populations. There was no difference in IgA, IgG, lysozyme or lactoferrin concentrations. Both aliquots demonstrated equivalent