When Specific Pathogen-Free hens were infected with Salmonella enteritidis PT4 by direct administration into the crop, the age of the bird at infection was found to have an effect on both pathogenesis and antibody response. Birds at 20 weeks of age showed no adverse signs and developed high titres of antibodies of the IgM class, while those which were 1 year old at infection developed relatively little antibody and had acute septicaemia, with 6 of 10 birds either dying or having to be humanely destroyed. The implication of these results for the control of salmonella infections in poultry is discussed.