Five strains of Salmonella enteritidis phage type 4 (PT4) isolated in 1978, 1984 and 1988 were examined for their ability to colonise the caecum and invade the liver of day-old chickens. All strains were capable of caecal colonisation and there were no differences in their colonisation ability in this respect. In contrast there was a gradation in the ability of strains to invade the liver, with strains isolated in 1988 proving the most invasive. Absence of a 38 megadalton (Md) plasmid, which has been shown to be involved in the virulence of S. enteritidis PT4 for BALBc mice, had little effect on the ability of strains of this phage type to colonise the caecum or invade the liver of day-old chickens. These results suggest that recent isolates of PT4 may have enhanced virulence for chickens which is not necessarily associated with the carriage of a 38 Md plasmid.