Addition of the iron chelators 2,2'-dipyridyl, deferoxamine mesylate or apo-transferrin to culture media affected the composition and the morphology of Pasteurella multocida cells. Cells grown under iron-restricted conditions expressed iron-regulated proteins and, in addition, iron deprivation markedly reduced the amount of capsular material covering the cells of P. multocida. The addition of iron neutralized the effect of these chelators on capsule production. Cells of P. multocida grown under iron-restricted conditions were more labelled by gold particles coated with polymyxin which is known to interact with the lipid A-core region of lipopolysaccharides, and showed increased affinity for porcine respiratory tract mucus than cells grown under iron-sufficient conditions. Bacterial cells grown in vivo in peritoneal chambers in rats were also only covered by a thin layer (15-20 nm) of capsular material. Although the capsule is believed to be an important virulence factor, our data indicate that under iron-restricted conditions, such as those encountered in vivo, P. multocida may not be heavily encapsulated.