The aim of this study was to investigate skin immunopathology following gene gun delivery of plasmid-encoding interleukin 3 (pIL-3) and hence explore the possible mechanisms of its adjuvant activity. Using the sheep as the experimental model, expressible pIL-3 was administered to the epidermis and the dermal/epidermal junction and its effects on the skin were assessed by histopathology, immunohistology and quantitative RT-PCR for a range of pro-inflammatory and immune response polarizing cytokines. Delivery of both functional and non-functional plasmids caused an acute inflammatory response with the infiltration of neutrophils and micro-abscess formation; however, the response to pIL-3 was more severe and was also associated with an early (24 h) infiltration of B cells and a later accumulation of CD172a-/CD45RA+ dendritic cells (DC). In terms of cytokine transcript expression, an early TNFalpha response was stimulated by gene gun delivery of plasmid-associated gold beads, which coincided with an immediate infiltration of neutrophils. However, only pIL-3 triggered the short-lived expression of IL-3 (peaking at 6 h) and significant long-term increases in both TNFalpha and IL-1beta. pIL-3 did not affect the expression of the immune response polarizing cytokines, IL-10 and IL-12.