Burns wound sepsis is not only the most common but also the most severe complication following extensive thermal injury. One conceivable explanation of this problem is a reduced capacity of the polymorphonuclear neutrophil leucocytes of these patients to combat the invading microbes. Fifty patients (42 male and 8 female) with deep dermal burns, covering 20-90 per cent of the total body surface area, were investigated from immediately after the injury until death or until healing of the wounds. The following functions of the neutrophil granulocytes were studied: chemotaxis and random migration utilizing a modified Boyden chamber technique, phagocytosis of Staph. aureus and IgG-coated latex particles, bactericidal capacity, e.g. killing of Staph. aureus and the neutrophil granulocyte content of: myeloperoxidase, lactoferrin, and chymotrypsin-like cationic protein. The presence of stimulators and inhibitors of the granulocyte functions was studied using gel filtration of the patient's serum on Sephacryl gel columns. Sera from all patients obtained within the first 1-3 days post-burn contained significantly increased amounts of heat-labile chemokinetic stimulating activity. Sera obtained between days 4 and 10 after injury contained significantly decreased amounts of heat-stable chemokinetic stimulating activity. Reduced chemokinetic activity was found during the third and fourth weeks following major burns (greater than or equal to 40 per cent) due to the presence of one or both heat-stable chemokinetic inhibitory activities. During the second week post-burn patients with burns larger than 40 per cent of the body surface area who showed an inhibition of chemotaxis, also had defects in phagocytosis, and often impaired bactericidal capacity concomitant with lower contents than normal of the granular enzymes. A hyaluronic acid preparation in low concentrations was found to counteract the migration inhibitory effect demonstrated in vitro in sera from patients with severe burns. Based upon these results a series of patients with severe burns and impaired functions of the neutrophil granulocytes have been treated with small amounts of this hyaluronic acid preparation subcutaneously. Very promising results have been noticed, similar to those found in vitro.