Isogenic strains of Salmonella enterica serovar Gallinarum biovar gallinarum harbouring RFLP variants of the virulence plasmid and the parent strains, were given orally to 1-week-old White Leghorn cockerels. Differences in viability and pattern of behaviour of the strains in the tissues were observed during the course of infection. Viable counts from the spleen and liver, the development of haematological changes in the form of anaemia, and pathological lesions were shown to be correlated. None of the RFLP variants of the virulence plasmid in particular were associated with the development of the anaemia. Viable counts of approximately 10(4) CFU in spleen and liver were required for the development of significant pathological- and haematological changes. The results showed that the severity of the haematological changes was closely correlated to the number of bacteria in the tissues, and thereby associated with the particular strain's ability to survive and multiply systemically.