Tumour necrosis factor-α (TNFα) and polymorphonuclear neutrophils play key and interrelated roles in the inflammatory response against infectious agents. However, these entities can mediate significant tissue damage if their biological activity becomes deregulated. We have previously shown that canine hyperimmune frozen plasma (HFP) contains anti-TNFα activity that is attributable to elevated levels of soluble TNFα receptor 1 (sTNFR1). The aim of this study was to determine the effect of HFP on TNFα levels and neutrophil infiltration in a lipopolysaccharide (LPS)-mediated rat air pouch model of inflammation. Rats were administered either HFP, HFP which had been pre-incubated with anti-sTNFR1 antibody (5 ng/ml), fresh frozen plasma (FFP), physiological saline (PS) at 2 ml/day or Carprofen at 5 mg/kg for 3 days prior to LPS challenge. Pouch fluid was withdrawn at 1, 6, 12, 24 and 48 h post-LPS challenge and assayed for TNFα by ELISA, and for total leukocytes and neutrophils by microscopic examination. At 6 h post-LPS challenge, both TNFα levels and neutrophil counts were significantly lower in HFP-treated rats than was found in FFP, PS or Carprofen treated animals (p<0.05). In a sTNFR1 blocking experiment, incubation of HFP with anti-sTNFR1 antibody resulted in significant increases in neutrophil numbers and TNFα levels, which suggests that the anti-TNFα activity observed in HFP may be due to elevated levels of sTNFR1. The data also revealed a significant inverse correlation between total leukocyte counts and sTNFR1 levels present in pouch fluid (r= -0.73, p<0.0001). Our observations suggest that HFP warrants further investigation as a possible means for modulating acute inflammatory processes where TNFα is a key mediator.