Strains of yellow fever virus isolated since 1927 in Africa and the Americas, and strains derived from them, have been differentiated by the responses of mice of different ages to intraperitoneal (i.p.) or intracerebral (i.c.) infection. Infection, antibody conversion, protection and death have been presented on age-dose response phase diagrams that serve as in vivo 'fingerprints' for the differentiation of virus strains and their modifications through passage and selection. Correlations between marker characteristics are discussed in terms of the efficiency of infection, regulatory (pre-challenge) and protective (post-challenge) immunity, and the expression of virulence. The requirement in virus strain specification for the resolution of events on pathogenic and immunogenic pathways is discussed.