Bacterial products fmet-leu-phe (FMLP), muramyl dipeptide (MDP) and lipopolysaccharide (LPS) were assayed for their ability to alter the inflammatory response to lambda carrageenan-induced pleurisy in Hooded Surgery rats. Continuously infused FMLP, or one initial i.v. dose of FMLP, MDP or LPS either ablated or partially suppressed the pleurisy. Total circulating leucocytes and neutrophils were suppressed by 55-65% when compared to the normal circulating leucocyte response to carrageenan pleurisy, excepting the protocol incorporating a single i.v. dose of FMLP where suppression was intermediate at 30%. There were also significant changes in the expression of FMLP receptors on circulating neutrophils. MDP and LPS induced a receptor number increase of 2 and 1.7 times initial value respectively, whilst a continuous FMLP infusion caused a receptor decrease to 0.3 times the initial value. The introduction of bacterial products at an alternative site to that of the pleurisy had an anti-inflammatory effect and the pleurisy was reduced.