Six commonly used strains of lymphocytic choriomeningitis virus (LCMV) [Armstrong (Arm) CA 1371, Arm E-350, WE, UBC, Traub and Pasteur C1PV 76001] were examined for distinctive genetic and biological properties. Agarose gel electrophoresis yielded no detectable differences among the L or S RNAs of these six strains. The RNase T₁ fingerprint patterns of LCMV Arm CA 1371 and E-350 RNAs were similar, but in contrast, those of the WE, UBC, Traub and Pasteur strains differed from each other and from the pattern of LCMV Arm CA 1371 and E-350. There were also differences among LCMV strains in their biological properties. LCMV Arm CA 1371, E-350 and Pasteur caused severe vasculitis and focal necrotizing hepatitis in the livers of neonatally infected BALB/WEHI mice in contrast to LCMV WE which caused minimal lesions. LCMV Arm CA 1371 and E-350 were lethal for neonatal C3H/St mice. In contrast, LCMV WE, Traub and Pasteur induced persistent infections in C3H/St mice. Adult guinea-pigs resisted infection by Arm CA 1371, E-350, Traub and Pasteur but succumbed to WE and UBC LCMV strains. Our results show a wide variation in the RNA genomes of LCMV strains commonly used in research laboratories, and these genomic differences are accompanied by variations in the biological properties of LCMV strains.