Commercially reared chickens were challenged with Salmonella enteritidis phage-type (PT) 4 by aerosol, or via the conjunctiva. Inhalation of 2.9 x 10(2) or 4.2 x 10(3) S. enteritidis resulted in the production of IgG antibodies to the lipopolysaccharide (LPS) of S. enteritidis PT4. When the aerosol inoculum was increased to 2.4 x 10(5) bacteria per bird the antibody produced were predominantly of the IgM-class. Chickens challenged with 10(3) S. enteritidis PT4 via the conjunctiva mounted only a poor immune response. Increasing the challenge dose to 10(8) S. enteritidis resulted in the production of high-titre serum antibodies of both the IgG and IgM classes. Results from this study suggest that aerosols containing small numbers of S. enteritidis PT4 might be responsible for intraflock infection of poultry.