Nocardia asteroides complex organisms derived from human specimens between 1979 and 1992 were identified to the species level. Of 117 N. asteroides complex organisms, 34 (29%) were N. farcinica, 28 (24%) were N. nova, and 55 (47%) were N. asteroides sensu stricto. An analysis of the specimen sites from which the organisms were derived showed that isolates derived from blood, brain, or bone marrow were more likely to be N. farcinica than the other two species. A study of the virulence of ten strains of each species was undertaken, using a mouse model with intravenous inoculation. The 50% lethal doses (LD50) for N. farcinica were significantly lower than those of the other two species. LD50 values for N. nova and N. asteroides were not significantly different. The above data confirming the greater virulence of N. farcinica support the identification of species within the N. asteroides complex.