The outer-membrane protein (OMP) profile of Pasteurella haemolytica grown under iron-replete and iron-restricted conditions was studied by polyacrylamide gel electrophoresis and immunoblotting. A serotype 1 isolate induced the synthesis of a new 77,000 Mr OMP in iron-restricted media while two other proteins of 100,000 Mr and 71,000 Mr were synthesized in increased amounts. None of these proteins were peptidoglycan-associated or heat-modifiable, and only the 100,000 Mr protein showed some degree of disulphide cross-linking. Kinetic analysis revealed that the iron-repressible proteins appeared in the outer membrane within 15 min of establishment of iron-restricted conditions. Analysis of P. haemolytica isolates representing serotypes 1 to 12 showed that iron-repressible OMPs of 77,000 Mr and 71,000 Mr could be induced in all 12 serotypes but that there was some variability in the expression of the 100,000 Mr protein. Immunoblotting of OMPs with convalescent sera from P. haemolytica-infected calves indicated that antibodies directed against all three iron-repressible OMPs were present, suggesting that these proteins were expressed in vivo.