The biological characteristic in mice and [African green monkey kidney Vero human larygeal carcinoma HEp-2, baby hamster kidney BHK-21 and primary mouse embryo] cell cultures of 7 strains of West Nile (WN) virus isolated in Nigeria were studied. The pattern of virus development in most organs of mice infected with 2 of the 7 strains tested was identical, while it varied with the remaining 5 strains. All 7 strains of WN virus multiplied with a cytopathic effect (CPE) in the 4 cell cultures examined.