Changes in virulence of Crimean-Congo haemorrhagic fever (CCHF) virus toward intracerebrally inoculated suckling mice (SM) were studied in relation to different host-passage histories. Two CCHF virus strains, one isolated from a human and the other from a tick, were passaged through various vertebrate and tick hosts and then reisolated and tested for their virulence toward SM. In various experiments, SM were inoculated with 12 different viral suspensions, each having a specific passage history. Survival curves of SM, which may reflect differences in viral strain characters, were established using an actuarial life table; differences were evaluated with the log-rank test. Regardless of the origin of the strain, CCHF viruses exhibited pathogenicity when passaged among SM. However, virulence, as measured by the proportion of deaths in SM, was altered following passage through another vertebrate host or tick. The final host seems to have a major influence on virulence. Because CCHF virus strains appear to vary little in their antigenic characters, it is hypothesized that hosts can induce phenotypic changes that modulate viral virulence.